These statistics represent the variance and standard deviation for each subset of data at the various levels of "x". If we can assume that the same phenomena are generating random error at every level of "x", the above data can be “pooled” to express a single estimate of variance and standard deviation. In a sense, this suggests finding a mean variance or standard deviation among the five results above. This mean variance is calculated by weighting the individual values with the size of the subset for each level of "x". Thus, the pooled variance is defined by