They are usually filmed in a laboratory. This means other activity, that might distract a baby, can be controlled. Also, using films means that observations can be recorded & analysed later, so it is unlikely that researchers will miss any key behaviours. Also, the filmed interactions means that multiple observers can record data & establish the inter-rater reliability of observations. Finally, babies don't know they are being observed, so their behaviour doesn’t change in response to. So the data collected has good reliability & validity.