This paper documents methodological lessons from experience in estimating yield and incremental production benefits from the Malawi Farm Input Subsidy Programme (FISP). Critical issues raised concern difficulties in obtaining reliable estimates of smallholder maize production, areas and yields with and without fertiliser. Comparison of methods and findings across a number of studies suggests that there is a significant upward bias (or overestimate) in area estimates obtained by relying on information from farmers on the areas of their crops. Use of GPS technology appears to provide an affordable but more reliable alternative method for measurement of plot areas, but further investigation is advisable regarding its accuracy and possible bias in measuring small plot areas. Estimates of yield from information on total plot harvests are affected by bias in area measurement and, for given estimates of production per plot, over-estimates of area lead to under-estimates of yield. However these yield estimates are also dependent upon the accuracy of farmer estimates of plot production, and these are potentially prone to (a) errors in farmer estimates of production and harvest, (b) unwillingness of farmers to reveal total harvest, and (c) errors in reporting harvests in standard units. While (b) is likely to lead to under-estimates of harvest and yield, (a) and (c) might lead to over- or under- estimates of harvest. It is not clear what overall bias this might lead to. The main alternative method of yield estimation (enumerators harvesting yield sub plots) is costly and widely considered to lead to over-estimates of yield.
Estimates of crop yield response to fertiliser application are not affected by bias in plot area estimation if yield is estimated from reported whole plot harvest. However fertiliser responses are over- (under-) estimated if whole plot harvest is over-(under-) estimated by farmers. Fertiliser responses are also over-estimated if yield sub plots over-estimate yield, and this is exacerbated if plot areas are over-estimated, as this leads to an under-estimate of fertiliser application rate.
Over-estimates of fertiliser yield responses are also likely if analysis does not allow for the effects of early planting, plant density, seed type, number of weedings and early weeding: these management practices raise yields but tend to be correlated with fertiliser use. Analysis of subsidy impacts needs to separate out the effects of these practices and also estimate how far management practices are changed by receipt of subsidised inputs.
The following recommendations are highlighted regarding area and yield survey methods:
•IHS2 estimates of crop areas and yields should be treated with caution as they are likely to have over-estimated areas and under-estimated yields
• Further work is needed to investigate the accuracy of GPS area measurement for small plots, but GPS methods are preferable to farmer estimates of plot areas
• Continued work is needed to develop accurate methods for estimating plot yields
• Investigation of fertiliser yield responses may need to rely on formal trials to address problems of multi-collinearity across management practices
• Due attention should be paid to both inter cropped and pure stands in analysis and reporting of yield and area estimates