Core Data stddev Query

NSExpression *minExpression = [NSExpression expressionForFunction:@"stddev:" arguments:@[ [NSExpression expressionForKeyPath:@"amount"] ]];
    
NSString *stdKey = @"std";
NSExpressionDescription *stdExpressionDescription = [[NSExpressionDescription alloc] init];
stdExpressionDescription.name = stdKey;
stdExpressionDescription.expression = minExpression;
stdExpressionDescription.expressionResultType = NSFloatAttributeType;

__block NSDictionary *dictionary = nil;
    
[context performBlockAndWait:^{
  NSFetchRequest *fetchRequest = [...]
  fetchRequest.propertiesToFetch = @[ stdExpressionDescription ];
    
  NSError *fetchError = nil;
  NSArray *result = [context executeFetchRequest:fetchRequest error:&fetchError];
  [...]
}];