请教各位老师：我在外刊里看到的一篇文章中有这样一句话（黑体字），意思能理解，但不懂这句话是什么结构或什么句式？

Now, I’m not one of those people who think G.D.P. is a terribly flawed or useless statistic. It’s a number we need for many purposes. But on its own it isn’t an adequate measure of economic success. There are a number of reasons this is true, but one key issue is that it tells you only what’s happening to average income, which isn’t always relevant to how most people live.