Correlation Coefficients for a Study with Repeated Measures

Repeated measures are increasingly collected in a study to investigate the trajectory of measures over time. One of the first research questions is to determine the correlation between two measures. The following five methods for correlation calculation are compared: (1) Pearson correlation; (2) correlation of subject means; (3) partial correlation for subject effect; (4) partial correlation for visit effect; and (5) a mixed model approach. Pearson correlation coefficient is traditionally used in a cross-sectional study. Pearson correlation is close to the correlations computed from mixed-effects models that consider the correlation structure, but Pearson correlation may not be theoretically appropriate in a repeated-measure study as it ignores the correlation of the outcomes from multiple visits within the same subject. We compare these methods with regard to the average of correlation and the mean squared error. In general, correlation under the mixed-effects model with the compound symmetric structure is recommended as its correlation is close to the nominal level with small mean square error

Reliability of Surface Electromyography During Maximal Voluntary Isometric Contractions, Jump Landings, and Cutting

The reliability of electromyographic (EMG) data has been examined for isometric and slow dynamic tasks, but little is known about the repeatability of this data for ballistic movements. The purpose of this study was to examine the within-session, trial-to-trial reliability of a variety of quadriceps and hamstrings muscles during isometric and ballistic activities. Data were analyzed by way of intraclass correlation coefficients (ICC), intersubject coefficients of variation (CVinter), and intrasubject coefficients of variation (CVintra). Twenty-four subjects performed 3 repetitions each of 2 randomly ordered test exercises, including landing from a depth jump (J) and cutting after a 10-m sprint (C). Data were acquired and processed with root mean square EMG for the muscles assessed, and data were analyzed for each exercise using a repeated measures analysis of variance. Results revealed that all ICC values were greater than 0.80, with most values greater than 0.90, CVinter values ranged from 5.4% to 148.7%, and CVintra values ranged from 11.5% to 49.3%. This study indicates that EMG is a reliable method for assessing the reproducibility of both the quadriceps and hamstrings muscle activation during either isometric or ballistic exercises

Intersession Reliability and Within-Session Stability of a Novel Perception-Action Coupling Task

BACKGROUND: The perception-action coupling task (PACT) was designed as a more ecologically valid measure of alertness/reaction times compared to currently used measures by aerospace researchers. The purpose of this study was to assess the reliability, within-subject variability, and systematic bias associated with the PACT. METHODS: There were 16 subjects (men/women = 9/7; age = 27.8 +/- 3.6 yr) who completed 4 identical testing sessions. The PACT requires subjects to make judgements on whether a virtual ball could fit into an aperture. For each session, subjects completed nine cycles of the PACT, with each cycle lasting 5 min. Judgement accuracy and reaction time parameters were calculated for each cycle. Systematic bias was assessed with repeated-measures ANOVA, reliability with intraclass correlation coefficients (ICC), and within-subject variability with coefficients of variation (CVTE). RESULTS: Initiation time (Mean = 0.1065 s) showed the largest systematic bias, requiring the elimination of three cycles to reduce bias, with all other variables requiring, at the most, one. All variables showed acceptable reliability (ICC > 0.70) and within-subject variability (CVTE <20%) with only one cycle after elimination of the first three cycles. CONCLUSIONS: With a three-cycle familiarization period, the PACT was found to be reliable and stable

Application of Multi-Market Contact Teory to Inter-Airline Rivalry : Case of FSC vs. Lcc in the US

The minimum number of days of pedometer monitoring needed to estimate valid average weekly step counts and reactivity was investigated for older adults with intellectual disability. Participants (N 5 268) with borderline to severe intellectual disability ages 50 years and older were instructed to wear a pedometer for 14 days. The outcome measure was steps per day. Reactivity was investigated with repeated measures analysis of variance, and monitoring frame was assessed by comparing combinations of days with average weekly step counts (with intraclass correlation coefficients [ICCs] and regression analyses). No reactivity was present. Any combination of 4 days resulted in ICCs of 0.96 or higher and 90% of explained variance. The study concludes that any 4 days of wearing a pedometer is sufficient to validly measure physical activity in older adults with intellectual disability

Correlation coefficients between longitudinally measured markers in pediatric liver transplant candidates with biliary atresia

Biliary atresia (BA) is the most common pediatric liver disease leading to liver transplantation during childhood, with very poor prognosis if untreated. In this study, we aimed to apply a linear mixed effect (LME) model to estimate the correlation coefficients among longitudinally measured total serum bilirubin, international normalized ratio (INR) for prothrombin time, and serum albumin, the three important prognosis predictors of pretransplant mortality. The dataset was obtained from the Standard Transplant Analysis and Research (STAR) of the United Network of Organ Sharing (UNOS). The primary analysis cohort consists of 1,700 pediatric liver transplant candidates who started their liver transplant waiting list between February 27, 2002 and June 24, 2010 with at least one follow-up measurement and had primary diagnosis of biliary atresia at the time of listing. In applying the LME model, we estimated the longitudinally measured markers via two different correlation structures: autoregressive of order one (AR1) and compound symmetry (CS) in rearranged data by a 7-day equally spaced repeated measures interval. Under the AR(1) structure, the estimated total correlation coefficients between total bilirubin and INR, total bilirubin and albumin, and INR and albumin were 0.4151, -0.2404, and -0.206, respectively, whereas the partial correlation coefficients (within-subject correlation) were 0.0656, 0.0916, and -0.0451, respectively. Under the CS structure, the estimated total correlation coefficients were 0.4307, -0.2432, and -0.1912, respectively and the partial correlation coefficients were 0.1742, -0.0678, and -0.0509, respectively for the above analysis. AR(1) structure had a better fit based on the Akaike information criterion (AIC) and the Bayesian information criterion (BIC). Several sensitivity analyses were conducted to understand the stability of the estimated overall correlation. The magnitudes of the estimates obtained from different sensitivity methods do not differ substantially. Public health significance: For two repeatedly measured markers, the total correlation, the between-subject correlation with time-averaged values, and partial correlation for within-subject measurements will provide a more complete picture of the correlations for these markers. Correlation by stacking all measurements of a subject together or between-subject correlation with time-averaged values is a measurement ignoring time effects and could either over or under estimate the total correlation coefficients. The estimated correlations between any two markers measured repeatedly for patients awaiting liver transplantation will give physicians a tool to analyze the relationship between two markers for patients during the waitlist period and may further help physicians understand disease progression and refine treatment strategy for candidates prior to receiving a transplant

Reproducibility of an isokinetic eccentric muscle endurance task

Despite the widespread use of isokinetic eccentric muscle endurance protocols, no research has examined the absolute and relative reliability of such tests. The aim of this study was to examine the reliability of an isokinetic eccentric knee muscle endurance task. Fifteen healthy untrained males volunteered to take part in the study and written informed consent was obtained. The procedures received approval from the University Research Ethics Committee. Participants visited the laboratory on three separate occasions: 2 weeks before testing to familiarize them with the experimental procedures, and on two separate occasions, 2 weeks apart and at the same time of day. Isokinetic eccentric knee extension and flexion movements of the dominant limb were performed using a calibrated dynamometer (Biodex System 3). Range of motion during testing was set using voluntary maximal full extension (0 rad) to 1.57 rad of knee flexion and testing was conducted at 1.56 rad . s(-1). Participants performed four maximal efforts to determine maximal peak torque. After 2 min rest, participants completed the all-out endurance test of 50 continuous eccentric repetitions. Data were gravity corrected and windowed to only include constant velocity periods. Repeated-measures analyses of variance were used to examine differences in maximal peak extension and flexion torque and the maximal torque measurement recorded during the endurance test. Absolute and relative reliability of the torque fatigue index, work fatigue index, and total work were assessed through calculation of intra-class correlation coefficients, coefficients of variation, and absolute 95% limits of agreement using the methods described by Bland and Altman (1986). Differences between the strength test and the endurance trial for peak torque were found to be non-significant for both quadriceps (323 vs. 323 N . m) and hamstrings (183 vs. 178 N . m). The intra-class correlation coefficients revealed significant (P<0.05) positive moderate to strong correlations (r=0.44-0.94) across repeated trials for all parameters except hamstring torque fatigue (P=0.11) and hamstring work fatigue index (P=0.08). Coefficients of variation ranged from 6% to 48% and were large for the work fatigue index and torque fatigue index but acceptable for total work for both the extensors and flexors. The 95% limits of agreement indicated systematic bias in repeated trials for both the work fatigue index and torque fatigue index for extensors and flexors, ranging from -3% to -10%, with less fatigue evident in the second test. There was also systematic bias for total work, with more work being performed during the second than the first test for both extensors and flexors. The random error was large for all variables and there was greater random error in the hamstrings compared with the quadriceps. Together with moderate to strong intra-class correlation coefficients and large coefficients of variation, the data suggest that there is small systematic bias in repeated eccentric muscle actions for both the quadriceps and hamstrings, although the random error was large despite habituation procedures. Therefore, data from an isokinetic eccentric muscle endurance task should be viewed with some caution even when participants receive considerable habituation before assessment

Correcting for Measurement Error in Segmented Cox Model

Measurement error in the covariate of main interest (e.g. the exposure variable, or the risk factor) is common in epidemiologic and health studies. It can effect the relative risk estimator or other types of coefficients derived from the fitted regression model. In order to perform a measurement error analysis, one needs information about the error structure. Two sources of validation data are an internal subset of the main data, and external or independent study. For the both sources, the true covariate is measured (that is, without error), or alternatively, its surrogate, which is error-prone covariate, is measured several times (repeated measures). This paper compares the precision in estimation via the different validation sources in the Cox model with a changepoint in the main covariate, using the bias correction methods RC and RR. The theoretical properties under each validation source is presented. In a simulation study it is found that the best validation source in terms of smaller mean square error and narrower confidence interval is the internal validation with measure of the true covariate in a common disease case, and the external validation with repeated measures of the surrogate for a rare disease case. In addition, it is found that addressing the correlation between the true covariate and its surrogate, and the value of the changepoint, is needed, especially in the rare disease case

A preliminary study on the reliability of physical performance measures in older day-care center clients with dementia.

BACKGROUND: Decline in physical functional ability is an intrinsic component of the dementia syndrome. Reductions in muscle mass and strength represent a major factor in the loss of functional ability Although resistance exercise has been studied as a method for maintaining/recovering function in populations of frail older adults, people with dementia have been systematically excluded because of uncertainty about the reliability of outcome measurements. OBJECTIVE: The purpose of this study was to determine the test-retest reliability of a battery of established performance-based measures of strength and function among subjects with dementia. SETTING: A hospital-affiliated adult day-care facility. SUBJECTS: Twelve of 28 older subjects with dementia of various etiologies were available for two assessments prior to implementation of a resistance-exercise intervention. METHODS: Subjects underwent an assessment of lower extremity strength and physical function consisting of two recorded trials of bilateral isometric strength of the knee extensor, hip flexor, and dorsiflexor muscles, as well as hand grip strength; repeated chair stands, evaluation of usual- and maximal safe-gait speed over a 6-m course, and the Timed-Up-and-Go Test. The entire assessment was repeated approximately 1 week later. An average of the trials for each measurement was computed for each of the two assessment periods, and intraclass correlation coefficients (ICCs) for these paired measurements were estimated using STATA. RESULTS: ICCs ranged from .56 for left iliopsoas to .77 for left dorsiflexors among the strength measures whereas measures of function ranged from .80 for number of steps in usual gait to .95 for time of fast gait. CONCLUSIONS: Performance-based measures of strength and function can be reliably assessed in older people with dementia, although measures of function appear to be more reliable than measures of strength

Am J Ind Med

ObjectiveTo develop a fatigue assessment scale and test its reliability and validity for commercial construction workers.MethodsUsing a two-phased approach, we first identified items for the development of a Fatigue Assessment Scale for Construction Workers (FASCW) through review of existing scales in the scientific literature, key informant interviews (n=11) and focus groups (3 groups with 6 workers each) with construction workers. The second phase included assessment for the reliability, validity and sensitivity of the new scale using a repeated-measures study design with a convenience sample of construction workers (n=144).ResultsPhase one resulted in a 16-item preliminary scale that after factor analysis yielded a final 10-item scale with two sub-scales (\u201cLethargy\u201d and \u201cBodily Ailment\u201d).. During phase two, the FASCW and its subscales demonstrated satisfactory internal consistency (alpha coefficients were FASCW (0.91), Lethargy (0.86) and Bodily Ailment (0.84)) and acceptable test-retest reliability (Pearson Correlations Coefficients: 0.59\u20130.68; Intraclass Correlation Coefficients: 0.74\u20130.80). Correlation analysis substantiated concurrent and convergent validity. A discriminant analysis demonstrated that the FASCW differentiated between groups with arthritis status and different work hours.ConclusionsThe 10-item FASCW with good reliability and validity is an effective tool for assessing the severity of fatigue among construction workers.20152016-02-01T00:00:00ZK01 OH010485/OH/NIOSH CDC HHS/United StatesT32 AR055885/AR/NIAMS NIH HHS/United StatesT42 OH008416/OH/NIOSH CDC HHS/United StatesU60 OH009762/OH/NIOSH CDC HHS/United States25603944PMC4677770871

Reliability and Validity of the Star Excursion Balance Test for Patients with Chronic Patellar Instability

The Star Excursion Balance Test (SEBT) is an eight-direction, maximal-reach balance test whose measurement properties are unknown in participants with chronic patellar instability. We designed an observational study with repeated measures to evaluate the test-retest reliability, cross-sectional and longitudinal construct validity, sensitivity to change and responsiveness of the SEBT in this population. Fifteen patients completed the SEBT and reported outcomes at baseline and two weeks and four patients completed testing three months later at the Fowler Kennedy Sports Medicine Clinic. Intra-class correlation coefficients (ICC) for the SEBT were fair to good, ranging from 0.66-0.84. The SEBT demonstrates good cross-sectional construct validity and we are unable to comment with certainty on longitudinal construct validity; correlations between SEBT reach distance and patient-reported outcomes showed agreement with our hypotheses in 93 of 126 (74%) and 46 of 108 (43%) directions. These are preliminary results of a larger continuing study; therefore definitive conclusions cannot be made