Determining the Reliability and Validity and Interpretation of a Measure in the Study Populations

For proper interpretation of study results, further reliability assessment is required to determine the variability from repeated samples from the same individual, and variation among individuals. Repeated samples from the same individual will indicate if the measure varies with time of day, menstrual cycle, or consumption of food or liquids. The extent that this impacts the reliability will dictate if the protocol should stipulate timing of specimen collection. The dynamics of colonization of human body sites by microbes are essentially unknown. Currently there are few estimates in the literature of how frequently there is a change in the bacterial strains (or other colonizing microbes) that commonly colonize the human gut, mouth, vaginal cavity, and skin. Also unknown is the average duration of carriage. There are some estimates for group B Streptococcus; colonization is very dynamic, with an average duration of carriage of ∼14 weeks among women. This suggests that assessments of reliability must be done over a short time, and that loss over 2-week period could as easily reflect true loss as sampling error.