How to perform Sensitivity analysis?

[PolashBanerjee]

I am performing a composite impact map of a socioeconomic impact assessment for a development project. I have used Multicriteria decision making method (i.e AHP) to find out the weight of 15 attributes. I have used a simple raster calculator to calculate the composite impact as:

I = W1*F(x1)+ W2*F(x2)+ ..........+WnF(xn)

where, Wi is the weight of i-th attribute and F(xi) is the attribute map for  x-ith attribute. If I want to perform SA by increasing and decreasing 10% of the attribute, which one should I manipulate, is it the attribute value or the attribute weight?  Please note that, W1 + W2 + ...... + Wn = 1. Hence, if I increase/ decrease the sum total of weights will be > or < 1, which violates the above condition. 

[DanPatterson_Retired]

If the weights must sum to 1, then you have to account for an offset in one variable with an appropriate increase in one or more of the others.  You can examine the affect by removing one variable and proportionally increasing the other weights to maintain sigma W, or you can decrease one or more but increase the other proportionally.  That is a constraint of the current model .... the summation of weights to 1