A sample that is arrived at by selecting sample units such that each possible unit has a fixed and determined probability of selection.
The term “representative”, as it is commonly used, is undefined in the statistical or mathematical sense; it means simply that the sample resembles the population in some way. The use of probability sampling will not ensure that any single sample will be “representative” of the population in all possible aspects. A common fallacy lies in the unwarranted assumption that, if the sample resembles the population closely on those factors that have been checked, it is “totally representative” and no differences exist between the sample and the universal or reference population.... representative sample
A population being studied because they are conveniently accessible (for example, all the people at a certain hospital or attending a particular support centre). As they are not a random sample of the whole population, they may be unrepresentative.... convenience sample
The tendency for the estimated magnitude of a parameter (e.g. based upon the average of a sample of observations of a treatment or intervention effect) to deviate randomly from the true magnitude of that parameter. Random variation is independent of the effects of systematic biases. In general, the larger the sample size, the lower the random variation of the estimate of a parameter. As random variation decreases, precision increases.... random variation / random error
are analysed by staining, in which Abnormal area dyes are used to Normal to show up structures or identify constituents such as antibodies or enzymes. A tissue sample may be tested with specific antibodies in the investigation of infection and inflammation. In some cases, a tis-sue culture may be required.... biopsy samples
a subgroup of a total population selected by a random process ensuring that each member of the population has an equal chance of being included in the sample. It is sometimes stratified so that separate samples are drawn from each of several layers of the population, usually on the basis of age, sex, and socio-economic group. Selection is sometimes facilitated by identifying, in advance, certain units (e.g. towns or neighbourhoods) that will together represent the whole (a so-called sampling frame).... random sample
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