patients, and given the inclusion and exclusion criteria for some clinical trials, the sample may not be representative of the majority of patients with the condition or disease. In simple data sets, the units are in one-to-one correspondence with the data values. In more complex data sets, multiple measurements are made for each unit. For example, if blood pressure measurements are made daily for a week on each subject in a study, there would be seven data values for each statistical unit. Multiple measurements taken on an individual are not independent (they will be more alike compared to measurements taken on different individuals). Ignoring these dependencies during the analysis can lead to an inflated sample size or pseudoreplication. While a unit is often the lowest level at which observations are made, in some cases, a unit can be further decomposed as a statistical assembly. Many statistical analyses use quantitative data that have units of measurement. This is a distinct and non-overlapping use of the term "unit." Units of collection and analysis Statistical units are divided into two. They are: Unit of collection: Units in which figures relating to a particular problem are either enumerated or estimated. The units of collection may be simple or composite. A simple unit is one which represents a single condition without any qualification. A composite unit is one which is formed by adding a qualification word or phrase to a simple unit. Example: labour-hours and passenger-kilometer. Unit of analysis and interpretation: Units in term of which statistical data are analysed and interpreted. Example: ratios, percentage, co-efficient etc. See also Census tract Research subject Specimen Sample point Statistical model Unit of analysis Bibliography Design of experiments Pre-publication chapters are available on-line. Sampling Sampling (statistics)