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SEOmoz provides an API of it's Linkscape data that other companies, even individuals can use for their own purposes. Oftentimes companies buy this data and use it in their own reports. Sometimes they use all of the data, sometimes just a portion of it. Unfortunately, there's no good way for me to tell how Link Diagnosis uses the SEOmoz data. To save money and resources, some companies "cache" the data - which works fine as long as they update the cache regularly. I'm not saying this to disparage Link Diagnosis, I simply don't know enough about them. That said, you can be assured that Open Site Explorer always has the latest, freshest and most accurate OSE data. Every time you use OSE, fresh data is served with no long-term caching, so it proves itself a very reliable tool for webmasters.