Algebraic theorems and findings of the research prove that the equation of internal rate of return for building and loan associations has in the range of (-1;infinity) two roots or no roots at all. It leads to some interpretation problems. In case of two roots, evaluation of savings and lending efficiency is based both on their value and NPV sign in interest rate ranges set by the roots. In case of no roots, investment profitability is measured solely based on the net present value.

The lack of roots is accompanied by the negative net present value. In such a case saving in associations is unprofitable. According to NPV criterion, it results from long-term saving under a slow decrease of NBP rediscount rate and resignation from placing the amount of tax relief in time deposits. Unprofitability of saving in associations results inseparably from a customer's loss of his/her right to a tax relief.

For a depositor it is most profitable to accumulate funds for a minimum period of 3 years under a rapid reduction of the rediscount rate. Profitability of such an investment is independent from the amount of payments (for monthly payments of up to PLN 1,400). For payments lower than PLN 1,400, profitability goes down due to an annual limit of tax relief.

For a 3-year, most effective, saving period, differences in profitability of saving in associations of Bank Pekao S.A. and Bank Œlšski S.A. are minor and amount to about 2 percentage points in favor of the latter. However, given a substantial vulnerability of evaluation on research conditions, this difference should not encumber the judgement on the advantage of one association over the other.