At http://www.kandrsmith.org/RJS/Misc/hygrometers.html there is a lot of tests on various hygrometers, like dht22, si7021 etc.
A useful comment if you are measuring quite seldom can be found at;
namely, that upon "measurement" DHT22 actually returns the previous measurement from memory - then actually does a measurement and stores that in memory.
If you poll once per hour, you get one hour old data every time. Poll twice, or poll "often enough".