Locate the comparison first
Sleep duration looks like one number. In 20 medical interns, paper diaries tracked participant averages from ambulatory polysomnography closely, while the researchers also examined differences between individual nights. A strong correlation between averages cannot make every nightly reading interchangeable.[1]
The online diary was used by a separate group of 266 students. That boundary matters: agreement between the paper diary and a sleep measurement device does not itself validate the online version against that device in the same people. The defensible strength is a workflow that can flag and correct entry errors.[1]
Count the missing nights too
Students entered data on about 95 percent of days in the online system. The authors estimated that staff work for a 30-day record fell from about 3.5 hours with paper to about 45 minutes online. Those times come from different groups, yet they show how making missing and inconsistent entries visible can change the research workload.[1]
The one-time survey estimated 0.7 hours more sleep than the online diary on average; the wearable estimated 0.5 hours more. Neither difference alone identifies the correct tool for every person. Internet access and regular self-entry also limit the diary. A direct next comparison should report both measurement differences and the number of unusable nights.[1]