10.7.2 The constant elasticity model
a. Nature of the model
It was seen in Chapter 3 that the constant elasticity demand function was in the power form:
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whereas the linear model, featuring constant marginal effects, was in the form:
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In these models, only three marketing mix instruments are considered: price, advertising and distribution. It was also explained in Chapter 3 that the power model is more realistic than the simpler linear model for two reasons:
1 It involves non-constant marginal effects. This allows for the existence of diminishing returns to marketing effort.
2 It involves interactions between the elements in the marketing mix. This means that marginal effects depend not only on the level of the variable itself but also on the values of the other marketing mix variables.
Thus in the linear model the marginal effect of advertising is given by:
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Where as in the power model the marginal effect is given by:
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Since the value of Q is affected by the values of the other elements in the marketing mix, it can be seen that the marginal effect depends on the levels these other elements.
We can now assume a linear cost function of the form:
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where u represents a constant level of unit production cost. We can now apply the technique of partial differentiation of the profit function to obtain expressions for the optimal levels of price, advertising and distribution.
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Therefore
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This expression is basically a rearrangement of expression (8.15) which was obtained in finding the optimal price and mark-up in terms of the price elasticity. However, we are now also in a position to find the optimal level of advertising and distribution in a similar manner:
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Substituting (10.34):
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Since
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similarly to (10.34):
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It should be noted that the second-order conditions for optimality are not considered here, for reasons of brevity.
The question now is: how can these optimal levels of the marketing mix instruments be interpreted?
b. Interpretation of the model
Several interesting and not entirely intuitive conclusions arise from the above analysis. Assuming no interactions between firms, as discussed in the previous chapter, the following conclusions are the most important.
1. The optimal level of price is independent of the levels of the other marketing mix variables. This is in particular a surprising result, coming from expression (10.36). The explanation is that the other marketing mix variables, advertising and distribution, are essentially treated as sunk costs. However, it is possible that these other variables may be relevant in affecting the optimal price if they influence the price elasticity. This possibility is considered in the next subsection.
2. The optimal price appears to be independent of uncontrollable factors affecting demand. These include, for example, seasonal factors and the marketing mix of competitors. However, as in the above case, these other variables may affect the optimal price through their influence on price elasticity.
3. The optimal ratio of advertising to sales revenue can be calculated. This is performed by combining equation (10.36) and (10.37); the resulting ratio is given by:
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This means that advertising and price should be set so that the resulting advertising-to-sales ratio is equal to the ratio of the advertising elasticity to the price elasticity. The implication here is that firms should not simply use an arbitrary fixed ratio in order to determine advertising budgets; urthermore, the ratio should be adjusted if the firm suspects that either the advertising or price elasticities have changed.
4. The optimal ratio of advertising to distribution expenditure is equal to the ratios of their respective elasticities. This result is obtained by dividing (10.37) by (10.38), as shown below:
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This means that if, for example, a firm’s promotional elasticity is twice the level of its distributional elasticity, it should spend twice as much on promotion as on distribution. Again this is not entirely intuitive; some firms have reacted to having a higher elasticity by spending less on the relevant instrument, because they regard it as being unnecessary to spend so much in order to have the same effect.