MyTezosBaker has an efficiency column. I assume this is a measure of if a baker is online for baking and endorsing events, though I'm not really sure.
How is efficiency calculated? Can this be devined from https://tzscan.io?
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The simplest way to calculate efficiency and the one currently used by that site is the following formula:
(bakes + endorses) divided by (priority 0 bakes rights + endorse rights)
Using that formula you can actually get a greater than 100% efficiency when you bake a non priority 0 block.
For added complexity, you can decide to weigh the value differently depending on age of blocks baked because it might be more meaningful to you to know that a baker has been doing a better job very recently rather than over a long period of time. Additionally you might want to value bakes as more important than endorses.
The data to perform these calculations yourself is available at https://tzscan.io/