Homoscedastic response surfaces

In view of Problem 3.17 and what ever insight you might have on the shapes of response surfaces in general, what fraction of response surfaces do you think might be homoscedastic Why Is it possible to consider any local region of the response surface shown in Figure 3.7 to be approximately homoscedastic ... 

Replication is often included in central composite designs. If the response surface is thought to be reasonably homoscedastic, only one of the factor combinations (commonly the center point) need be replicated, usually three or four times to provide sufficient degrees of freedom for s. If the response surface is thought to be heteroscedastic, the replicates can be spread over the response surface to obtain an average purely experimental uncertainty. 

Is the response surface shown in Figure 12.6 homoscedastic or heteroscedastic ... 

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