The average means simply taking values, summing them, and then dividing the results by the number of total values you consider. Actually, this measure doesn't give any information about the size of values: if they are both positive and negative the average will be similar to zero! Therefore, if you are interested on size of values without regard for positive or negative, a different approach must be used. You can think of taking values, without considering their sign, and then doing the above mentioned average. That can be a kind of measure, but actually it is not the best in terms of statistics. In statistical terms, the mean is done by squaring the considered values, and then by squaring root the average (the above mentioned one) of them.
The average is better called "arithmetic mean", the rms is called "quadratic mean" instead. Their is a relationship between those kind of measures, i.e. QUADRATIC MEAN^2 = ARITHMETIC MEAN^2 + VARIANCE, where VARIANCE is the variance of that particular population of considered values.
thank you very much!
i have another question, when i would like to calculate the power (a constant voltage times a current with many harmonic frequencies), should i use "rms".
that's to say,how to include the power of harmonic frequencies of the non-ideal current.
thanks a lot!