calculation of the efficiency

[#1]

Hey everyone.

It seem simple to me (at least hope)

I make a calculation of the efficiency of production lines by multiplying the number of hours by lines.

And everything works fine but ...

When I added a new production line to the system in July, I get a funny situation.

In the months before I get a 90% efficiency
In the months from July receives an average of 90%.

But if I choose all year (January to August) I receive 70% efficiency as the system multiplying the X +1 lines for all the year. How can I set it to multiplying by X until June and July to the end of the year will be multiplying with X +1 lines

[#2]

Looks like you neeed a weighted mean. You need to record the number of lines for each month and weight the monthly efficiency by the monthly weight. IN your case it would be

.9X +.9X +.9X .9X +.9X +.9X +.9(X+1) +.9(X+1)+.9(X+1)+.9(X+1)+.9(X+1)+.9(X+1)
-----------------------------------------------------------------------------------------------------------
X+X+X+X+X+X+(X+1)+(X+1)+(X+1)+(X+1)+(X+1)+(X+1)

That would be for the whole year.

See http://en.wikipedia.org/wiki/Weighted_mean for a good description of a weighted mean.

Steve