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It's pretty common to see fluctuation like this from one Linkscape index to the next. Also, SEOmoz is constantly working to improve the index, and from time to time changes the way it crawls the web. Domain Authority is calculated fresh with each new update, and it's a comparative metric based on other sites on the web. This means it's better to compare your DA to your competition with each update, than to compare small fluctuation against yourself from month to month. To your point about OSE - to improve efficiency, OSE only records the first 25 links for every domain because after the first few links, each additional link makes very little difference in rankings or correlation data.