SEARCH · PubMed Health

Results for “Individual variability”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources

PubMed2005

Varieties of attention-deficit/hyperactivity disorder-related intra-individual variability.

Intra-individual variability in behavior and functioning is ubiquitous among children with attention-deficit/hyperactivity disorder (ADHD), but it has not been systematically examined or integrated within causal models. This article seeks to provide a conceptual, methodologic, and analytic framework as a foundation for future research. We first identify five key research questions and methodologic issues. For illustration, we examine the periodic structure of Eriksen Flanker task reaction time (RT) data obtained from 24 boys with ADHD and 18 age-matched comparison boys. Reaction time variability in ADHD differed quantitatively from control subjects, particularly at a modal frequency around .05 Hz (cycle length approximately 20 sec). These oscillations in RT were unaffected by double-blind placebo and were suppressed by double-blind methylphenidate. Together with converging lines of basic and clinical evidence, these secondary data analyses support the speculative hypothesis that the increased power of multisecond oscillations in ADHD RT data, and by inference, in attentional performance, represents a catecholaminergic deficit in the ability to appropriately modulate such oscillations in neuronal activity. These results highlight the importance of retaining time-series data and quantitatively examining intra-subject measures of variability as a putative endophenotype for ADHD.

Attention Deficit Disorder with Hyperactivity↗

PubMed2005

Metabolism-based drug-drug interactions: what determines individual variability in cytochrome P450 induction?

Individual variability in cytochrome P450 (P450) induction comprises an important component contributing to the difficulties in assessing and predicting metabolism-based drug-drug interactions in humans. In this article, we outline the major factors responsible for the individual variability in P450 induction, including variable transporter activity and metabolism of inducers in vivo, genetic variations of P450 genes and their regulatory regions, genetic variations of receptors and regulatory proteins required for induction, and different physiological and environmental elements. With a better understanding of the major determinants in P450 induction and a profile of the phenotypes of these determinants in each individual, it is believed that the individual variability in induction-mediated drug-drug interactions can be adequately evaluated.

Animals↗

PubMed1989

Small intra- and large inter-individual variability in lithium clearance in humans.

We report the inter- and intra-individual variability in fractional lithium clearance (CLi), an alleged quantitative index of Na and water delivery from the proximal tubules, in humans (N = 91). The inter-individual variability was large, the variation coefficients at various Na excretion rates ranging between 11% and 19%. The intra-individual variability was small, the relative intra-individual standard deviation for duplicate measurements (N = 33) being 5%. These observations suggest large inter-individual differences in proximal tubular Na reabsorption. To confirm this, we also studied the inter- and intra-individual variability in the maximum urine flow during water diuresis (Vmax), an index of Na delivery to the diluting segment. They were found to be almost identical to the inter- and intra-individual variability in CLi, and fractional CLi and Vmax correlated strongly (r = 0.83, P less than 0.001). In addition, the inter-individual variability in the fractional clearance of uric acid (CUA), a directional marker of Na reabsorption in the proximal tubules, was large, but the intra-individual variability small. The correlation between fractional CUA and CLi, however, was relatively weak (r = 0.40, P less than 0.01). Although our results do not prove the exact, quantitative validity of the lithium clearance concept, we conclude that the variability in CLi reflects large inter-individual differences in Na handling in the proximal segments of the nephron. Our observations also have implications for the use of the lithium clearance method. The large inter-individual variability in CLi makes the method less suitable to detect subtle differences in CLi in small, unpaired groups of subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1995

Individual variability in responses to nicotine.

Individual variability in acute responses to nicotine, which may be defined as variable magnitude of effects following controlled dosing, is generally attributed to stable characteristics of tobacco users such as genetic/constitutional factors or to chronic behavioral factors (e.g., long-term use of other drugs). Often overlooked, however, is that such variability may also be due to the transient influence of the situational factors in which people consume nicotine, such as acute stress or physical activity. Results of selected studies from the author's laboratory provide examples of each of these sources of variability in nicotine responding on subjective, behavioral, and physiological measures. All studies used a nasal spray method of nicotine dosing or controlled smoking (paced puffing) to control acute nicotine exposure, an essential methodological feature of any research on individual differences in acute responses to nicotine. As an example of genetic/constitutional factors, gender differences in nicotine responding have begun to receive some attention, with few differences emerging. However, females may be more responsive than males to nonnicotine stimuli associated with smoking (e.g., sight and taste of smoke). In terms of chronic behavioral factors, long-term use of nicotine produces attenuation of most subjective and some behavioral effects of nicotine, reflecting chronic tolerance, and the possibility that chronic use of other drugs may alter responses to nicotine (i.e., cross-tolerance or cross-sensitization) deserves greater study. Of particular emphasis in this review is the modulating influence of acute situational factors on nicotine responding. Human studies have shown that magnitude of nicotine's subjective effects may depend on the predrug subjective state, level of physical activity vs. rest, and concurrent acute intake of other drugs, among other situational factors. Proper consideration of these situational factors may reveal the greatest source of individual variability in responding to nicotine and clarify the impact of more stable genetic/constitutional or chronic environmental factors.

Arousal↗

PubMed1979

Individual variability in cortical localization of language.

Individual variability in the localization of language, as measured by object-naming, was assessed for left lateral peri-Sylvian cortex with a multi-sample technique of stimulation mapping at a constant current. This study was performed during craniotomy under local anesthesia in 10 patients with medically intractable epilepsy and the usual pattern of left brain dominance for language. A high degree of variability in the exact location of naming was present. Only one area, a small portion of the third frontal convolution immediately in front of the motor strip, showed naming changes in all patients in whom it was sampled. This area is considerably smaller than the classical Broca's area. Elsewhere in language cortex, including all parts of the posterior language area, there was considerable individual variability. Because of this, the classical model of language localization is an inaccurate basis for establishing the risk of aphasia in surgical therapy of dominant hemisphere peri-Sylvian lesions in this and related patient populations. Rather, that risk should be assessed from the individual localization of language, established by the multisample technique of stimulation mapping.

Adolescent↗

PubMed2001

Employment of intra-individual variability to improve computerized ECG interpretation.

One of the reasons for the limited practical utility of computer programs for interpretation of electrocardiograms (ECGs) is their susceptibility to intra-individual variability. Two of the most prominent sources of intra-individual variability in ECGs, electrode placement variations and respiration, were studied for their effects on computerized ECG interpretation. Previous research has shown that the effects of intra-individual variability on computerized ECG interpretation depend largely on the individual ECG. To enable the assessment of chest electrode position variations for individual standard 12- lead ECGs, ECGs resulting from simulations of such position variations were interpreted. Variability due to respiration was assessed by interpreting all individual ECG beats instead of an averaged beat. In this paper two methods are presented that employ information about the intra-individual variability in individual ECGs. The first method provides an estimate of the reliability of the interpretation, the second attempts to improve the interpretation itself. In the first method we quantified the variation in interpretation caused by the two sources of intra-individual variability with the use of a stability index, a high index value indicating a low variation in interpretation. This index was subsequently studied using two sets of ECGs. For the first set a â clinical' reference interpretation was obtained from discharge letters. For the second set three cardiologists provided a â cardiologists' reference. The performance of subgroups of ECGs having stability indices higher than a particular value was computed. It appeared that for the â cardiologists' reference, the interpretations of ECGs with a high stability index were more often correct. No effect was found for the â clinical' reference. In the second method we attempted to improve the original interpretation by combining the alternative interpretations into a new interpretation. This was done by taking the median or the average of the quantified alternatives. These combined interpretations proved to perform better than the original interpretation when a cardiologist's interpretation was taken as a reference. This paper shows that intra-individual ECG variability can be used to improve original interpretations. This can be done without having to record multiple ECGs, provided that a model is available to simulate intra-individual variability. The presented methods do not depend on the classification algorithm that is used. They can be used both during classifier design to correct imperfections, and in routine use of the classifier to produce more representative classifications.

Decision Trees↗

PubMed2006

Intra-individual variability in behavior: links to brain structure, neurotransmission and neuronal activity.

Intra-individual variability reflects a transient, within-person change in behavioral performance. It is a common component of aging-related cognitive decline and the behavioral changes associated with neurodegenerative and other brain-related disorders such as traumatic brain injury and schizophrenia. Behavioral changes within an individual can reflect alterations at a systems or a cellular level in the brain, and monitoring intra-individual variability can therefore provide a warning of underlying pathology. Despite frequent reports of intra-individual variability, there is little synthesis, and no direct examination of the neural underpinnings. Here, we integrate seminal findings from cognitive research across lifespans of individuals, and also neuropsychological and neurobiological findings, to identify key questions and some potential answers, and to set challenges for fostering future research into intra-individual variability.

Aging↗

PubMed1999

Long-term intra-individual variability of the background EEG in normals.

OBJECTIVES: Forty-five healthy adult volunteers underwent repeated qEEG examinations with retest intervals 25-62 months in order to investigate the long-term intra-individual variability of several qEEG features such as, absolute and relative power, power asymmetry, coherence, mean and peak frequency and entropy. Prior to any computations all parameters were transformed to Z-scores on the basis of a normal database. METHODS: Correlation coefficients were used to test the effect of the time on the test-retest differences. Correlation coefficients were also computed between baseline and retest values, as a measure of intra-individual stability, to make our results comparable to most literature data. By computing the standard deviations for test-retest differences, the intra-individual variabilities of the examined parameters were obtained in the unit of inter-individual variability of normal population. The same calculations were carried out with values obtained from the odd and even numbered epochs of the same EEG sections. This way, that portion of the intra-individual variability was estimated that might be introduced even by chance only when the epochs were selected randomly from the same section of EEG conforming to selection criteria. RESULTS: As for our results, further increase of test-retest differences with time after 25 months might be so insignificant that it could not be demonstrated in our test material. The long-term intra-individual variability for most parameters, especially for total absolute power and alpha mean frequency, was less than the inter-individual variability in the normal population. The moment-to-moment variability was least in the case of the absolute power. CONCLUSIONS: Estimates for intra-individual variability expressed this way in Z-scores might easily be used in the follow-up of patients even for a few years.

Adult↗

PubMed2004

Chromosomal aberrations and in vitro radiosensitivity: intra-individual versus inter-individual variability.

In order to assess the applicability of the micronucleus (MN) and G2 assays as biomarkers of in vitro radiosensitivity and cancer susceptibility, we investigated the inter- and intra-individual variation of these endpoints. For the MN assay unstimulated blood cultures from 57 healthy donors were exposed in vitro to 3.5 Gy Co gamma-rays and for the G2 assay PHA stimulated cultures were irradiated with a dose of 0.4 Gy Co gamma-rays in the G2 phase of the cell cycle. For 14 donors, 2-15 repeat samples were tested over a period of 3 years. The repeat experiments revealed that the intra-individual variability was not significantly different from the inter-individual variability for both G2 and MN assays. As the intra-individual variability determines the reproducibility of the assay, our results highlight the limitations of these endpoints in detecting reproducible differences in radiation sensitivity between individuals within a normal population. Due to the high intra-individual variability and no significant difference with the inter-individual variability found in our study we conclude that care has to be taken when results obtained with chromosomal aberration assays based on one blood sample are used to assess the individual radiosensitivity. Multiple blood sampling may be necessary to draw reliable conclusions. Although more validation studies on the reliability of the G2 and MN assay will be required before they can be used in a confident way as biomarkers of individual radiosensitivity or cancer susceptibility the assays are very valuable to examine population radiosensitivity and the relationship between radiosensitivity, cancer predisposition and genotype.

Adult↗

PubMed2006

Inter-individual variability of the facial morphology during conscious smiling.

INTRODUCTION: Since many simulation and animation procedures for mimicry are based on averaged values and do not take into account an individual's situation, the goal of this study was to investigate the intra- and inter-individual variability of conscious smiling. MATERIALS AND METHODS: Twenty-three surface points were marked on the faces of 31 adult test individuals. Six photographs of each subject's face were then taken under standardized conditions, one with a neutral facial expression and five while the individual was asked to smile (conscious smile "on command"). After digital superposition of the individual images (neutral and smiling) the displacements were measured for every measurement point on each individual. All data underwent statistical evaluation after normalization in the vertical and horizontal directions. Evaluation of intra- and inter-individual variability was carried out by analyzing the corresponding coefficients of variation and the formation of confidence intervals (confidence level 95%). RESULTS: The repeated measurements upon conscious smiling showed a moderate intra-individual variability with coefficients of variation under 10%. In contrast, the inter-individual variability of the measured values was relatively high. Here variation coefficients were measured of between 28% and 60% depending on the localization of the measuring point. The displacements' absolute magnitude was greatest in the lower facial third. The two corners of the mouth, the upper lip in the philtrum area, the cheeks and the side wings of the nose revealed the highest displacements. CONCLUSION: Due to the high inter-individual variability, an analysis based on averaged values or a simulation of the conscious smile must be viewed with a certain amount of criticism.

Adult↗

PubMed2005

[Assessment of individual variability of human heart rhythm].

A new approach is represented in the article to assess individual variability of heart rhythm in healthy subjects. The approach has been maintained based on the theory of adaptation and homeostasis. Homeostatic range of circulatory system reactions has been determined using ortho- and clinostatic functional exertions. Averaged duration of 5-minute cardio-intervals of ECG recording was determined using statistical analysis. The homeostatic range limits of averaged cardio-intervals may be considered as the limits of individual variability of heart rhythm. Assessment of individual variability of heart rhythm may be used to assess whether psychophysical state of a person is ready to professional activity.

Adult↗

PubMed1992

Inter- and intra-individual variability in food intake of elderly people in Perugia (Italy).

It has been shown that each individual has a considerable day-to-day variation (intra-individual variation) in his or her level of food consumption. A large intra-individual variation has adverse effects on the reliability of research studies. The effect of the intra-individual variation can be minimized by taking food intake records over several days. An increase in the number of days entails higher costs, and this could limit the practicability of the study. In the recent literature on the methodology of dietary surveys, there is a growing interest in the estimation of the number of days required to conduct a reliable dietary survey. Recent developments in statistical theory allow the problem of large intra-individual variability to be overcome. These new statistical techniques require knowledge of the intra- and inter-individual variability and the appropriate adjustment of the statistical results. The aim of the present study was to evaluate the number of days of recorded intake required to obtain an estimate of the components of variance (the inter-individual and the intra-individual variability) focusing on foods rather than nutrients.

Aged↗

PubMed2006

Intra-individual variability of saccadic velocity measured with the infrared reflection and magnetic scleral search coil methods.

BACKGROUND: The infrared (IR) and the magnetic scleral search coil (MSC) systems for eye tracking were studied with regard to the intra-individual variability in saccadic eye movement recordings. METHOD: Three healthy subjects performed similar saccadic eye movement tasks at five different occasions with both the IR (Orbit XY-1000) and the MSC (Skalar Medical) techniques. The maximum velocity (VMAX) and slope constant (C) of the main sequence plots were analyzed with regard to the coefficient of variation (CV) and the intraclass correlation coefficient (Ricc). In addition, the possible reasons for variability in the IR recordings, especially different causes for noise, were analyzed and discussed. RESULTS: The main sequence data showed intra-individual variation with both recording systems, but the coefficient of variation was higher for VMAX with the IR compared to the MSC method. Ricc analysis showed that 36% of the variance of VMAX and 49% of the variance of C resulted from intra-individual variability in recordings of the IR system. The corresponding results for the MSC recordings regarding VMAX and C were 48% and 88%. CONCLUSIONS: Saccadic eye movement recordings yielded a larger intra-individual variability with the IR system than with the MSC system. The effect that the MSC annulus may have on the ocular motor command signal and the possible low pass filter caused by the coil slipping on the surface of the eye may partly explain the relatively lower velocity in the MSC recordings. Also, noise in the IR recordings induces peaks of eye velocity, which can be reduced considerably by filtering. However, the variability in the recordings, which was larger in the IR than in the MSC recordings, did not seem to be decreased by filtering. The basic level of noise in the recordings was not clearly associated with the amount of reduction of VMAX when the IR recordings were filtered. We suggest that artefacts of the saccadic signal, which can be related to changes in the reflecting surface of the eyes and eyelids, are important factors for explaining the variability and high-velocity peaks in the IR recordings. Lighting conditions was confirmed as a cause for noise, but temperature and air humidity changes in the goggles were not suspected to influence data in the normal experimental setting. Although noise, shortcomings of the recording technique and procedure may offer explanations for the intra-individual variability, the calibration procedure and changes in attention and fatigue of the subject should also be considered.

Adult↗

PubMed1979

[Individual variability and an error in the method of determining protein thermostability].

The individual variability of the heat resistance of two enzymes (aldolase and actomyosine) of the m. gastrochemius of the grass frog Rana temporaria has been quantitatively evaluated by means of analysis of variance. 37--41% of the total variability of the character may be due to errors in the methods of investigation, whereas 59--63% of the variability is due to phenotypic differences between individuals of the same population. The extent of error is primarily associated with the initial stages of the experiment, i. e. the isolation of the enzyme preparations. The individual variability of the heat resistance of proteins is lower than that of the cells or the organisms from which they are derived.

Analysis of Variance↗

PubMedDate not supplied

Inter and intra individual variability of acute insulin response during intravenous glucose tolerance tests.

To analyze the inter- and intra-individual variability of acute insulin response to intravenous glucose (AIRG), 41 healthy volunteers underwent an intravenous glucose tolerance test (IVGTT) and 29 of them a second IVGTT 1 to 9 months later. Basal glycemic, insulin (IRI), and C-peptide values were similar for both IVGTTs. Different indices were used to estimate AIRG. A great inter-individual variability of AIRG (CV around 60%) was detected. AIRGs were not statistically different between the two IVGTTs, and the within-subject variation was fair at the group level (CVs approximately 30%). However, individual coefficients of variation ranged from 2 to 60% between the two tests. Moreover, subjects considered as "low" responders during test 1, returned to "normal" values during test 2. Conversely, other subjects dropped to a "low" response in IVGTT 2. Insulin peak (IRI max) occurred between 1 and 3 minutes after glucose infusion in 85% of the control population, but time points of IRI max were different for 45% of the population between the two IVGTTs. These results suggest that AIRG during IVGTTs are reproducible at the group level, but that AIRG has to be interpreted with caution in individual early detection of pre-insulin-dependent diabetes mellitus because inter- and intra-individual variability could be high even for some normal subjects.

Adult↗

PubMed2006

Within- and between-individual variability in estimated energy expenditure and habitual physical activity among young adults.

OBJECTIVE: To examine the within- and between-individual variability in the estimated total energy expenditure (TEE) and habitual physical activity (PA) in free-living young adults. We were specifically interested in the day-to-day variation. DESIGN: Cross-sectional. SETTING: University. SUBJECTS: A total of 277 (125 males, 152 females) participants aged 18-24 years. INTERVENTIONS: None. MAIN OUTCOME MEASURES: Subjects recorded their daily PA with the Bouchard 3-day diary. Descriptive statistics for TEE, moderate-to-vigorous physical activity (MVPA), inactivity (IA), and activity energy expenditure (AEE) were calculated and presented as a frequency distribution. Within- and between-individual variability were tested by RMANOVA and also presented as the coefficient of variation (CV). RESULTS: Mean values for TEE, MVPA, AEE, and IA were comparable between males and females; however, considerable between-individual variability existed. Only MVPA showed a significant difference between sexes (P<0.05), due to slightly greater vigorous PA in males compared to females (8.0 and 6.8 kcal/kg/day, respectively; P=0.12). Results from the RMANOVA revealed no significant day-to-day variability in any TEE or PA variable and the CVs were not significantly different between males and females. TEE and IA displayed mean CVs of approximately 12% while mean CVs for AEE and MVPA approximated 32 and 84%, respectively. CONCLUSIONS: Results from this study suggest (1) a limited day-to-day variability in TEE and (2) a large degree of between-individual variation for TEE and PA exist. This study highlights the importance of understanding the variability associated with TEE, which is important for PA measurement and proper PA and nutritional recommendations. SPONSORSHIP: None.

Adolescent↗

PubMed2004

Inter- and intra-individual variability in performance near the circadian nadir during sleep deprivation.

The study purpose was to assess inter- and intra-individual variability in neurobehavioral function near the circadian nadir during sleep deprivation and conduct exploratory factor analyses to assess relationships among alertness and performance measures during sleep deprivation. Twenty-five healthy individuals (16 females) aged 18-25 years participated. Participants were sleep deprived for two nights under controlled laboratory conditions using a modified constant routine procedure. A comprehensive battery of neurobehavioral performance tests, subjective sleepiness (SSS), and objective alertness (MWT) were assessed. Seventeen of the 22 neurobehavioral measures were impaired by sleep deprivation (all P < 0.01). The use of multiple neurobehavioral performance measures revealed impairments for all individuals during sleep deprivation. However, sleep deprivation effects were task dependent within and between individuals. Gender contributed minimally to inter-individual variability in performance. Exploratory factor analysis reduced the 22 measures to seven independent factors. Our findings indicate that no individual was especially vulnerable or resistant to the performance impairing effects of sleep deprivation. Instead, inter- and intra-individual variability in performance during sleep deprivation was task dependent. The finding that subjective sleepiness and objective alertness were not related to any performance measure during sleep deprivation suggests that these measures may assess independent brain functions.

Adolescent↗

PubMed2002

Fibrinogen: intra-individual variability in patients with arterial disease and in patients with cardiac valve replacements.

BACKGROUND: Elevated fibrinogen, considered an independent risk factor for coronary disease, stratifies an individual as high risk for coronary disease. A risk marker requires little intra-individual variability during a long period. OBJECTIVES: To establish intra-individual variability of fibrinogen levels in patients with coronary disease. METHODS: We investigated fibrinogen levels prospectively in four blood samples drawn from 267 patients with a history of arterial disease (arterial group) and from 264 patients with cardiac valve replacements (valvular group). The samples were taken during the course of 78.7 and 78.8 days from the arterial and valvular groups respectively. RESULTS: Marked intra-individual dispersion with a reliability coefficient of 0.541 was found in the arterial group and 0.547 in the valvular group. The Bland-Altman test showed low probability to obtain similar results in different samples from the same individual. These results show large intra-individual variability, with similarities in the arterial as well as in the valvular group. CONCLUSIONS: It is not possible to stratify a patient by a specific fibrinogen dosage.

Analysis of Variance↗