Testing Whether Two Slopes Are Significantly Different

To test whether the slopes of two different straight lines are equal, use the following equation  

The pooled estimate of the standard deviation of the slope is given as follows, provided the number of data points is large  

If the numbers of data points for each line, ni and ri2, are not large, then you must use the following equation to calculate the pooled estimate of the standard deviation  

The variance can easily be obtained using the Excel function VAR. Equation 11-16 for the pooled estimate of the standard deviation is implemented, for example, by using the Excel formula 

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