In statistics, random samples are used to make generalizations, or
inferences, about a population. If your data are not collected randomly, your
results may not represent the population. For more information, go to
Randomness
in samples of data.
Each observation should be independent from all other observations
For observations to be independent, the value of a particular
observation does not depend on any previous observation. If your observations
are not independent, your results may not be valid. For more information, go to
How are dependent and independent samples different?.
The sample size should not be small
If your sample size is small, the resampling results may be
unreliable. To ensure that your results are valid, collect a medium to large
sample. An adequate sample size depends on the characteristics of the data. Use
the histogram to determine whether your sample size is large enough.