To compare apples with apples, I'd like one survey that studies two continents.
To quote Dr House, "Everybody lies". A survey is valueless in determining how religious a country is, it can only tell you how religious the people polled
want you to think they are.
A better way to measure religious belief is to count attendance at churches, temples and mosques.
Excluding special events (Easter and Christmas, Eid, Passover, Hannukah, etc.), how many people go to services during a normal, non-special week?
Take a look at any English church in a typical Sunday, and you will find seating for a few hundred, occupied by a half-dozen octo- and non-genarians, being addressed by a priest from Kenya or Nigeria (because almost no English born people have wanted to enter the priesthood since the 1950s).
Sure, they can get a full house on Christmas Day or Easter Sunday. But even that is only possible because so many churches have closed, and so many parishes have been merged, since the mid 20th Century.
People go to those services, and to weddings, funerals, and baptisms, because they don't want to disappoint gran, or like to think of themselves as "culturally Christian", rather than because they are devout believers.