Measuring educational attainment

I hope folks find this useful.

I wanted to see how to measure educational attainment. I got educational data from the 2016 edition of World Development Indicators http://data.worldbank.org/ . I used an average of the indicators over 2008-2014, looking at three indicators:

Gross enrollment ratio, secondary, both sexes (%)
Gross enrollment ratio, tertiary, both sexes (%)
Adult literacy rate, population 15 years, both sexes (%)

The data base has 215 total countries. However,
77 countries have no data on literacy
48 countries have no data on tertiary enrollment
36 countries have no data on secondary enrollment

Next, I looked at correlations among education indicators
Secondary and Tertiary enrollment = 0.76
Secondary enrollment and literacy = 0.83
Tertiary enrollment and literacy = 0.64

It seems that secondary enrollment could be a fair substitute for literacy rate. More countries have data, and the two correlate reasonably well.

I mentioned using an average of the indicators over 2008-2014. This seemed necessary since indicators may not have data for every year. For example, for literacy rate:

Only 44 countries have data for 2010
Only 55 countries have data for 2013

Thanks.