sub inaccuracy {
  my ($base, $multiple, $log) = @_;
  
  my $inaccuracy = 0;
  my $interval = 0;
  
  foreach my $sale (@sales) {

    $interval++;

    $inaccuracy +=
     ( $sale -
       $base -
       $multiple * log ($interval) / log ($log)
     ) ** 2;

  }

  return $inaccuracy;
}