Lower bounds on inequality of opportunity and measurement error

•Lower bound estimates of inequality of opportunity can have substantial measurement error.•Lower bound estimates of inequality of opportunity are not likely to be comparable across samples.•Cross-country comparisons of lower bound estimates of inequality of opportunity can be misleading.

I show that lower bound estimates of inequality of opportunity can have substantial measurement error, and that measurement error can vary considerably across samples. As a consequence, the traditional cross-country comparisons researchers make can be misleading.