Data obtained with no systematic manipulation of key variables. <a href="search.pl?p=Regression analysis">Regression analysis</a> is particularly useful in handling such data as it assesses the partial effects of each variable by statistically controlling for other variables in the equation. If the variables do not vary or the <a href="search.pl?p=explanatory variables">explanatory variables</a> are highly correlated with one another, nonexperimental data cannot be used to estimate relationships.<o:p></o:p></P>