The intraclass correlation coefficient is then calculated using stata s icc command. Intracluster correlation coefficients in adults with diabetes in primary. Stata module to calculate intraclass correlation icc after. The intracluster correlation coefficient in cluster randomisation. Feb 24, 2014 in order to perform a welldesigned cluster randomized trial to demonstrate the effectiveness of usual care uc and cp in heart failure treatment, the intraclass correlation coefficient icc should be available before conducting a trial to estimate the required sample size. To provide information concerning the magnitude of the intraclass correlation coefficient icc for clusterbased studies set in primary care. We estimated the intracluster correlation and the coefficient of variation for a range of outcomes using data from five communitybased cluster randomised controlled trials in three lowincome countries. Icc intraclass correlation coefficients acronymattic. Intracluster correlation coefficients in the greater mekong. Clustered data effects on sample size and approaches to. Order stata intraclass correlations for multilevel models.
Get your free icc reliability calculator from mangold international. Second, our results accord with findings from previous methodological studies regarding the key determinants of the intracluster correlation coefficient. Intraclass correlations for multilevel models stata. When xtmixed is available, i plan go back and rework it, and add the additional iccs described in k. The intracluster correlation coefficient icc of the primary outcome plays a key role in the design and analysis of cluster randomized trials crts, but the precise definition of this parameter. It is depicted by the greek letter rho there are different methods of calculating the icc, usually requiring a pilot study, but all compare the variance within clusters with the variance between clusters. Determinants of the intracluster correlation coefficient in. To plan studies that have appropriate power, investigators need good estimates of clustering effects, typically in the form of intraclass correlation coefficients iccs. Stata module to calculate intraclass correlation icc after xtmixed. This study estimates the schoollevel intracluster correlation coefficient icc for rabies knowledge change following an educational intervention program.
Package icc august 29, 2016 type package title facilitating estimation of the intraclass correlation coef. Intraclass correlation coefficients for cluster randomized. Icc r1 intraclass correlation coefficient of r1 where r1 is formatted as in the data range b5. Patterns of intracluster correlation from primary care. Intracluster correlation coefficients for sample size.
In addition to estimation of icc, a hypothesis test is performed with the null hypothesis that icc r0. When the intracluster correlation is 1, everyone within a class acts the same, and so you effectively only have 100 independent observations. Estimating the intracluster correlation coefficient for. Sample size calculation formulas for cluster randomized trials are widely available 2,46. An intracluster correlation coefficient icc is commonly used to quantify how. The intraclass correlation coefficient icc is a measure of the reliability of measurements or ratings. Apr 25, 20 the intraclass correlation coefficient is then calculated using stata s icc command as described in figure 1 using variables for the measure, participant, and interviewer. Intraclass correlation coefficient icc file exchange. The statistical measure of this clustering effect is known as the intracluster correlation coefficient, or icc. Statas estat icc command is a postestimation command that can be used after linear, logistic, or probit randomeffects models. Icc refers to a number of quantities, but the simplest form is the usual linear pearson productmoment correlation among a set of pairs of values when the order in each pair is arbitrary. Study design and setting reanalysis of data from 31 clusterbased studies in primary care to estimate intraclass correlation coefficients from random effects models using maximum likelihood estimation.
This graph made in optimal design, shows how the power of a study for detecting a treatment effect of 0. Observed intracluster correlation coefficients in a cluster. It estimates intraclass correlations for multilevel models. In this paper, we describe the results of a survey to inform the appropriate reporting of the intracluster correlation coefficient icc the statistical measure of the clustering effect associated with a cluster randomized trial. The high value of icc shows there is a fair degree of agreement between the judges. The f value, degrees of freedom and the corresponding pvalue of the this test are reported. The following are supplementary data to this article. Estimating the intracluster correlation coefficient for evaluating an. It accounts for the relatedness of clustered data by comparing the variance within clusters with the variance between clusters. We fit a threelevel mixed model for gross state product using mixed. For the purpose of assessing interrater reliability and the icc, two or preferably more raters rate a number of study subjects. A survey of experts was conducted to generate a priori hypotheses of factors that might affect icc size.
Mar 20, 2015 the degree of correlation is usually measured by the intracluster correlation coefficient icc. Such intracluster correlations are typically small. Mar 20, 2015 few studies have comprehensively reported intracluster correlation coefficient icc estimates for outcomes collected in primary care settings. Sample size calculation formulas for cluster randomized trials are widely available 2, 46. The objective of this research was to identify determinants of the magnitude of intracluster correlation coefficients iccs in cluster randomized trials from the field of implementation research. This trial had an intervention aimed directly at the patient and an outcome measurement for which the variance between practices is low compared with. Intraclass correlation coefficient medcalc statistical software. This study estimates the schoollevel intracluster correlation coefficient icc for. Intraclass correlation coefficients iccs are frequently employed in health science research, often to assess intrarater and.
The intraclass correlation coefficient may be thought of as the correlation between any two observations made on the same subject. Lb and ub are upper and lower bounds of the icc with alpha level of significance. Stata because the focus of this article is on the general nature, selection. The intracluster correlation coefficient icc is a major parameter of interest in cluster randomized trials that measures the degree to which responses within the same. Jun 14, 2011 an intracluster correlation coefficient icc is commonly used to quantify how much more similar outcomes are for individuals within clusters than for those in different clusters 5, 6. Wong, forming inferences about some intraclass correlation coefficients. The intracluster correlation coefficient in cluster. The intraclass correlation coefficient icc, usually denoted. Obs covparm subject estimate bvar icc 1 intercept pid. The icc is defined as the ratio of the betweencluster variance to the total variance both between and within clusters, and therefore has a value between 0. Intracluster correlation coefficients for the brazilian. One column for the variable of interest, one column indicating which survey participant each row pertains to, and one column indicating which interviewer each row pertains to. In statistics, the intraclass correlation, or the intraclass correlation coefficient icc, is a descriptive statistic that can be used when quantitative measurements are. R package to estimate intracluster correlation coefficient.
Intraclass correlation coefficients typical of cluster. Table 1 intracluster correlation coefficient icc for prevalence of p. The similarity of observations within a cluster can be quantified by means of the intracluster correlation coefficient icc, sometimes also referred to as intraclass correlation coefficient. In this paper, a sas macro is provided to calculate the icc and its confidence limits. To account for intracluster correlation, sample sizes required for cluster randomized trials must be increased to reach the desired power. The icc is defined as the ratio of the betweencluster variance to the total variance both between and within clusters, and therefore has a value between 0 and 1. Intracluster correlation coefficients and coefficients of. Dec 22, 2004 estimates of the size of intracluster correlations come from post hoc examination of studies that have used either allocation or sampling by cluster and a number of intervention studies have published observed intracluster correlation coefficients 46. Computing intraclass correlation sas code fragments.
Intracluster correlation coefficients in cluster randomized. The icc for a particular outcome can be described as the amount of variation in the dataset that can be explained by the variation between clusters. An important characteristic of such studies is the presence of intracluster correlation, typically quantified by the intracluster correlation coefficient icc, that indicate the proportion of data variability that is explained by the way of clustering. The intracluster correlation coefficient icc is a measure of the relatedness, or similarity, of clustered data. Stata s icc can measure absolute agreement and consistency of agreement. The icc is variously defined as the pearson correlation between two members of the same cluster or the proportion of the total variance in the outcome attributable to the variance between clusters. An intracluster correlation coefficient icc is commonly used to quantify how much more similar outcomes are for individuals within clusters than for those in different clusters 5,6. Intraclass correlation real statistics using excel. Using data from a large primary care study, we aimed to. The application of the macro is demonstrated in a clinical reliability case study, the.
Boosting quality in science is our mission and reliability is a basic part of it. Assume that a different set of four judges is used to rate each target so that we have a oneway randomeffects model. This is our gift to the scientific community to allow everyone creating reliable results. Statas icc can measure absolute agreement and consistency of agreement. Apr 22, 2014 clusterbased studies in health research are increasing. It describes how strongly units in the same group resemble each other. Intracluster correlation coefficients and coefficients of variation for. The degree of correlation is usually measured by the intracluster correlation coefficient icc. The need for clear reporting of randomized trials has been widely recognised. Similarly, the need for appropriate standards of reporting of cluster trials is more widely acknowledged. Intraclass correlation coefficient for continuous data, icc often used to assess interrater reliability icc is the correlation between two measurements made on same subject corr. An introduction to intraclass correlation that resolves some. We can estimate the correlation of ratings made on the same targets by typing. Methods a crosssectional survey was conducted with 226 students from 7 schools in sikkim, india, using cluster sampling.
In statistics, the intraclass correlation, or the intraclass correlation coefficient icc, is a descriptive statistic that can be used when quantitative measurements are made on units that are organized into groups. May 01, 2004 the intracluster correlation coefficient, or the intracluster correlation coefficient icc,or. Stata s estat icc command is a postestimation command that can be used after linear, logistic, or probit randomeffects models. Consider a dataset containing 24 ratings of 6 targets by 4 judges.