Hi nizze,

sorry if I answer to your old post, but I have a curiosity regarding your question.

When you say baseline, you need that quantity for doing forecast (for example of commercial demand in a market)?

Anyway to calculate a baseline like "average value" you can assume that your data follows (or before you can do a test) a normal distribution. Then you can choose an alpha factor (normally a "realistic" choice could be 0,05) which you'll use for determining a confidence interval. For example for alpha=0,05, the interval is

[mean-1,96*std.dev.;mean+1,96*std.dev.].

https://en.m.wikipedia.org/wiki/File:NormalDist1.96.png
With normal hypothesis confirmed you'll have the 95% of probability to finding a new value included in that interval.

For your scope, you can use the interval for determining the "outlier" values (those out of interval).

Hope I was helpful to you (if you still need).