6 in itself need not be a source for great concem. Psacharopoulos (1985) shows social rates of return of around 10% even in the advanced countries.

Of greater concern is the suggestion that the cross-section studies contain many flaws leading to an upwards bias in the estimates (Behrman and Birdsall, 1987 and Behrman, 1990)'. Two studies in particular have attempted to correct for this bias. One important source of bias omitted from most studies is a measure of the quality of schooling (see also below). Behrman and Birdsall (1983,1985) explored this issue for a sample of males in Brazil. 'he sample studied exhibits a private rate of return of 20.5%, but adjusting for quality (proxied by length of schooling for teachers), the rate dropped to 11.0%. (The work of Card and Krueger (1992) discussed below also shows that the return to education is positively correlated with schooling quality.) A recent study by Glewwe (1991) using data from Ghana has also tried to overcome problems of bias in estimating the return to education. This work takes account of variation in cognitive ability and in school qualitys. He shows that, with the omission of variables that are positively correlated with years of schooling, such as school quality and family background, ordinary least squares estimators will be biased. His research is very critical of the estimation of rates of return to additional years of schooling. Despite the concems over his data voiced in footnote 5 below, there can be little quarrel with his airgument that the return arises to skills and not to schooling _per se._ He concludes that the private rate of return to education in Ghana is at most around 6% p.a. and suggests that 'rates of return to improvements in school quality" are needed. An implication of his finding is that other studies of rates of return may provide misleading policy recommendations about educational investment programs.

There have been a number of other studies looking at the effects of education quality, although most of these have been carried out in the United States. In 1966 the influential 'Bennet, Glennester and Nevison (1992) come to a similar conclusion for higher education in the United Kingdom. Their argument is that higher education is disproportionately taken up by people whose background would, in any case, tend to give them access to better-paid jobs.

sOne concern about the data used in Glewwe' **s** tudy is that they show the average duration of schooling to be 9.59 years. This compares with OECD figures for 1974 showing lower figures in Denmark, France, Germany, Greece, Italy, the Netherlands, Norway, Portugal, Spain and Sweden. Only Belgium, Canada, UK and USA show longer schooling duration. Nevertheless _World Development epoPw4 1991_ shows 40% of the population and _57%_ of the female population as illiterate. Even in 1988 only 73% of the relevant age group were enrolled for primazy education, with only 39% enrolled for secondary education. It would be interesting to know whether the survey results have been cross-checked with aggregate data on school attendance. Obviously any inflation of the number of years of education claimed by the respondents will tend to depress the estimated return. Literacy and numeracy, if measured as part of the survey are much less likely to be exaggerated.