Index: openacs-4/packages/accounts-finance/tcl/finance-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/accounts-finance/tcl/finance-procs.tcl,v
diff -u -r1.3 -r1.4
--- openacs-4/packages/accounts-finance/tcl/finance-procs.tcl	21 May 2010 23:36:33 -0000	1.3
+++ openacs-4/packages/accounts-finance/tcl/finance-procs.tcl	22 May 2010 00:17:17 -0000	1.4
@@ -233,15 +233,17 @@
  }
 
 ad_proc -private acc_fin::qaf_mirr { 
-    net_period_list 
+    period_cf_list 
     finance_rate
     re_invest_rate
     {intervals_per_year 1}
  } {
      Returns a Modified Internal Rate of Return
  } {
+     # see http://en.wikipedia.org/wiki/Modified_internal_rate_of_return
      # create separate positive and negative cashflows from list
-     set period_count [llength $net_period_list]
+
+     set period_count [llength $period_cf_list]
      foreach period_cf $period_cf_list {
          if { $period_cf > 0 } {
              lappend positive_cf_list $period_cf
@@ -253,6 +255,6 @@
      }
      set pv [acc_fin::qaf_npv $negative_cf_list [list $finance_rate] $intervals_per_year]
      set fv [acc_fin::qaf_fvsimple $positive_cf_list $re_invest_rate $intervals_per_year]
-     set mirr [expr { pow( -1. * $fv / double( $pv ), 1. / double( $period_count ) ) - 1. } ]
+     set mirr [expr { pow( ( -1. * $fv ) / double( $pv ), 1. / ( double( $period_count ) -1. ) ) - 1. } ]
      return $mirr
  }
Index: openacs-4/packages/accounts-finance/tcl/test/finance-test-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/accounts-finance/tcl/test/finance-test-procs.tcl,v
diff -u -r1.3 -r1.4
--- openacs-4/packages/accounts-finance/tcl/test/finance-test-procs.tcl	21 May 2010 23:36:33 -0000	1.3
+++ openacs-4/packages/accounts-finance/tcl/test/finance-test-procs.tcl	22 May 2010 00:17:17 -0000	1.4
@@ -21,6 +21,6 @@
         aa_equals "Check FV simple interest" [acc_fin::fvsimple $cashflow3 $reinvest_rate] "7600.0"
         aa_equals "Check NPV" [acc_fin::npv $cashflow1 $discount_rate] "0.000550114657813"
         aa_equals "Check IRR" [acc_fin::irr $cashflow1] $discount_rate
-        aa_equals "Check MIRR" [acc_fin::mirr $cashflow1 $finance_rate $reinvest_rate] "0.1791"
+        aa_equals "Check MIRR" [acc_fin::mirr $cashflow1 $finance_rate $reinvest_rate] "0.179085686035"
     }
 }