We’ve mentioned it before, but reminders never hurt.
Before you spend hours searching an online database, determine how complete the database is. Some sets of data include all records in a specific series. Others may be in progress, only including part of the time span the title covers. The webpage title may say the materials are from 1850 to 1950, with 1850-1855, 1870-1880, and 1940-1950 being included. Always read the details.