Why is it so difficult to calculate the number of people who speak English as a second language?

There are a number of reasons:

1. When does a person count as a second language speaker? How good does that person's command of English have to be? Is it enough to be able to successfully manage simple conversations? Or does the person need to be able to read and understand books in that language? There are no definite, agreed-upon answers to these questions.

2. For some countries, linguists have studied the sociolinguistic situation carefully and numbers are available. In other countries, no such data exists. The numbers that the linguist David Crystal in his book „The Cambridge Encyclopedia of the English Language“ arrived at, are in these cases based on the percentage of people over the age of 25 who have completed secondary education and who can be expected to have a certain command of English.