How do I analyze falling traffic

For the past 6 months, my traffic has been falling consistently for several of my sites and one increasing.

1. celebrity blog site, it's fallen from a peak of about 40k slowly to 13k (added about 60 new articles). Site has about 400 articles.
2. clipart site from 90-150k/month to 70k per month. i added about 1000 extra clip art during that time. Site has about 30k clip art images
3. A game cheats site from 27k to 43k per month with no new content. An increase but I didnt do anything!...

How do I analyze falling traffic

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