33 assessment of the supply response of agriculture. However, it is useful to keep in mind that most empirical works are based on a partial equilibrium set up. Non-price incentives, such as policies affecting infrastructure, roads, and so on, are also important.241 Studies of the supply response on SSA agriculture have discussed whether or not the African smallholder responds normally, quickly and efficiently to relative price changes, or whether the institutional constraints limit their response. Empirical evidence exists for SSA countries, assembled by Helleiner (1975) and Bond (1983), which shows that the response of single crop output to price incentives is high. Tables Al and A2 summarize the elasticity ranges for SSA countries. The evidence shows that farm producers in SSA respond to price incentives as any other farm producer in the developing world-they behave rationally.

FOOD CROPS **AND** CASH CROPS SUPPLY ELASTICITIFS. It is quite useful to distinguish between cash crops and food crops because of the large difference in the marketing magnitude of their inputs. Statistics are generally available for cash crops because farmers record the sale of their harvests when their output is either exported or processed in factories. In the case of food crops, however, traded production is often too small, making the availability of statistics inconsistent. In general, supply elasticities of food crops are low as can be inferred **by** Krishna (1963) from his study on single crop supply elasticities. In another, more recent study, Jaeger (1991) combined cross-section and time-series data for 43 SSA countries and assessed the responsiveness of prices, policies, natural disasters, and rainfall on the supply of exports and food crops. Natural disaster was represented by the percentage of population affected by disasters, rainfall was proxied with the residual from estimating a regression trend line for cereal yields.25/ In the short run, annual export crops price response was found to be elastic (above 0.9), and export supply elasticities with respect to **the** real exchange rate for annual crops

**241 Most of the litemturc on supply response in SSA dcaling with a single commodity includes non-price factors variables. For example, Olii (1969), Adesimi (1970), and Mnitha (1970), (sec the note** in **Table Al), anong others, include a time trend variable to represent eithcr technology or infrastructure, or population: the use of proxies for the weather variable are also common. However, there are** factors **that** influence price responsiveness which **are** not **considend** in these **models.** In particular, the level of prices **received** by **farmers may** not **be** as **important as** the timeliness of their receipt, or the political stability, or the rura **infrstructure.**

**25/ The use** of this **proxy** for **rainfall** was based on **the** assumption that for **most countries, variations** in **average** yields **will** result primarily **from variations** in weather. **Furthermore, for** the regression equation **it** is assumed that this proxy is not corrclated with the dependent variable. But, if this proxy is a fiintion of **the dependent** variabk, then **the** estimates **are** biased.