Monday, 4 March 2013

Repeated Measures



A repeated-measures design is one in which multiple, or repeated; measurements are made on each experimental unit. The experimental unit could be a person or an animal, and repeated measurements might be taken serially in time, such as in weekly systolic blood pressures or monthly weights. The repeated assessments might be measured under different experimental conditions.

Repeated measurements on the same experimental unit can also be taken at a point in time. For example, it might be of interest to measure the diameter of each of several lesions within each person or animal in a study. The dependency, or correlation, among responses measured in the same individual is the defining feature of a repeated-measures design. This correlation necessitates a statistical analysis that appropriately accounts for the dependency among measurements within the same experimental unit, which results in a more precise and powerful statistical analysis.

Repeated-measures analysis encompasses a spectrum of applications, which in the simplest case is a generalization of the paired t test.1 a repeated-measures within-subjects design can be thought of as an extension of the paired t test that involves ≥3 assessments in the same experimental unit. Repeated-measures analysis can also handle more complex, higher-order designs with within-subject components and multifactor between-subjects components. The focus here is on within-subjects designs.

Monday, 16 July 2012

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the meter, second or degree Celsius. The science of measurement is called metrology.

Monday, 19 September 2011

Repeated measures design

A repeated measures design refers to studies in which the same measures are collected multiple times for each subject but under different conditions. For instance, repeated measures are collected in a longitudinal study in which change over time is assessed. Other studies compare the same measure under two or more different conditions. For instance, to test the effects of caffeine on cognitive function, a subject's math ability might be tested once after they consume caffeine and another time when they consume a placebo.