Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_19948664_2 ( index__year_ VARCHAR, countries_sampled VARCHAR )

### Question ###
How many years were listed under index when there were 157 countries sampled?

### Accurate SQL ###

SELECT COUNT(index__year_) FROM table_19948664_2 WHERE countries_sampled = 157