One strength of Schaffer and Emerson's study is that it has external validity. Most of the observations were made by parents during ordinary activities and reported to researchers. The alternative would be to have observers present in the babies' homes,. This may have distracted the babies or made them feel more anxious. This means it is highly likely that he participants behaved naturally while being observed., However, mothers may be biased in what they reported which means even if the babies behaved naturally their behaviour may not have been accurately recorded