Controlling for socio-economic status

No two households will maintain exactly the same per capita income. Moreover, income and wealth are notoriously difficult to measure accurately. Assumptions underlying the concept of positive deviance are that (1) the level of income is more or less the same in very poor neighbourhoods; (2) fairly simple indicators, such as land ownership, housing, and visible possessions can be used to identify families living at approximately the same level of poverty; and (3) additional statistical adjustments made for income can more or less remove its confounding effects.

The studies classified as true positive-deviance research in table 2 did find some variability in family wealth, although they tried to compare homogeneous groups. Morley and associates (1968) concluded, for example, that the fathers of the well nourished were "better farmers" than those of the malnourished. Such variability may be critical to positive deviance when additional small amounts of income are earned by an enterprising mother and used to feed her child.

The point to be made here is that positive-deviance studies must make the attempt to control for socio-economic status. The Kanawati and McLaren (1973) study of failure-to-thrive in Lebanon, for example, was not included in table 2 because large socioeconomic differences were found between the malnourished and well-nourished groups. Given the difficulties of measuring income, total household expenditure is often used as a proxy variable for income and can be analysed by subcategory.