using the sample information to determine the factor–level combination that will
give the maximum yield. From the table, we see that crop yield increases when the
nitrogen application is increased from 40 to 60 (holding phosphorus at 10). Yield
also increases when the phosphorus setting is changed from 10 to 20 (at a fixed
nitrogen setting of 40). Thus, it might seem logical to predict that increasing both
the nitrogen and phosphorus applications to the soil will result in a larger crop
yield. The fallacy in this argument is that our prediction is based on the assumption
that the effect of one factor is the same for both levels of the other factor.
We know from our investigation what happens to yield when the nitrogen application
is increased from 40 to 60 for a phosphorus setting of 10. But will the yield
also increase by approximately 20 units when the nitrogen application is changed
from 40 to 60 at a setting of 20 for phosphorus?
To answer this question, we could apply the factor–level combination of 60
nitrogen–20 phosphorus to another experimental plot and observe the crop yield. If
the yield is 180, then the information obtained from the three factor–level combinations
would be correct and would have been useful in predicting the factor–level
combination that produces the greatest yield. However, suppose the yield obtained
from the high settings of nitrogen and phosphorus turns out to be 110. If this happens,
the two factors nitrogen and phosphorus are said to interact. That is, the
effect of one factor on the response does not remain the same for different levels of
the second factor, and the information obtained from the one-at-a-time approach
would lead to a faulty prediction.
The two outcomes just discussed for the crop yield at the 60–20 setting are
displayed in Figure 14.2, along with the yields at the three initial design points. Figure
14.2(a) illustrates a situation with no interaction between the two factors. The
effect of nitrogen on yield is the same for both levels of phosphorus. In contrast,
Figure 14.2(b) illustrates a case in which the two factors nitrogen and phosphorus
do interact.