//Alpha = Perf annualisée fond - B*perf anualisée (indice)
        private double AlphaPeriod(int period)
        {
            double A = VALUE_IF_ERROR, P = 0, Pi = VALUE_IF_ERROR;
            double B = 0;

            switch (period)
            {
            case 0:
                B  = beta;
                P  = perfAT;
                Pi = index.GetATPerf();
                break;

            case 25: B = beta3M;
                P      = perf3M;
                Pi     = index.Get3MPerf();
                break;

            case 50: B = beta6M;
                P      = perf6M;
                Pi     = index.Get6MPerf();
                break;

            case 100: B = beta1Y;
                P       = perf1Y;
                Pi      = index.Get1YPerf();
                break;

            case 300: B = beta3Y;
                P       = perf3Y;
                Pi      = index.Get3YPerf();
                break;

            case 500:  B = beta5Y;
                P        = perf5Y;
                Pi       = index.Get5YPerf();
                break;
            }
            if (B != VALUE_IF_ERROR && Pi != VALUE_IF_ERROR)
            {
                A = P - B * Pi;
            }
            return(A);
        }