This is one of those things with technology that if you're not comfortable doing this technique, make no attempt at it!
An LCD pixel is composed of three primary subpixels (RGB). In a 'dead' or 'dark' pixel none of the three subpixels can illuminate, most commonly for a misaligned transistor. Gently rubbing your finger over the display can sometimes align the subpixels enough to reilluminate them but more often than not lead to people ruining their LCDs for pressing too hard.
The opposite of this is commonly referred to as a 'hot' or 'stuck' pixel where one or more of the three subpixels remain constantly illuminated. The rub trick can sometimes work there as well but with limited degrees of success.
Often times manufacturers have a specified number or dead/stuck pixels that they will replace the display for under warranty. Before attempting to 'rub one out' check with the manufacturer, you'd be surprised how many of them will make an attempt to fix or replace the display.
You should allow access to the cumulative results for those who've completed the survey. If someone takes the time out to do a lengthy survey with a lot of thought involved, there should be some pay off...