The NPV decision rule needs that a company invest in all projects that have a positive net present value. This presumes that sufficient funds are available for all incremental projects which are only true in a perfect capital market. When inadequate funds are available that is when capital is rationed projects cannot be selected by ranking by absolute NPV. Selecting a project with a large NPV may signify not choosing smaller projects that in combination give a higher NPV. In its place if projects are divisible they are able to be ranked using the profitability index in order make the optimum selection. If projects aren't divisible different combinations of available projects should be evaluated to select the combination with the highest NPV.