Selection bias occurs when the association between exposure and health outcome is different for those who complete a study compared with those who are in the target population. Sources of selection bias. Selective survival and losses to follow-up After enrollment of subjects and collection of baseline data there is
NUMBER OF PUBLIC PHOTOS UPLOADED IN 2013. - 680 million, - 56.7 million per month in average, - 1.86 million per day in average. NUMBER OF PUBLIC PHOTOS UPLOADED IN 2012. - 556 million, - 46.3 million per month in average, - 1.52 million per day in average. NUMBER OF PUBLIC PHOTOS UPLOADED IN 2011. - 575 million, - 47.9 million per month in average, - 1.57 million per day in average.
More than 1 billion unique users visit YouTube each month. Over 4 billion hours of video are watched each month on YouTube. 72 hours of video are uploaded to YouTube every minute. 70% of YouTube traffic comes from outside the US. YouTube is localized in 53 countries and across 61 languages. In 2011, YouTube had more than 1 trillion views or around 140 views for every person on Earth.