Knee pain is self-reported based on 2 questions. Participants are first asked if they experienced pain or aching in any of their joints in most days for at least one month during the prior year. Those who answer affirmatively are then asked about location of the pain or aching.

Participants are asked the following questions:

1. Since the last interview on [last interview date], have you had any pain or aching in any of your joints on most days for at least one month?
2. Have you had pain in or around the knees including the back of the knee on most days for at least one month?

Notes: At baseline interview, question #1 reads: In the past year, have you had any pain or aching in any of your joints on most days for at least one month?

The provided reference describes the baseline version of this variable.