Iterating over object properties
I can't really judge if it's a good idea, but one way could be creating an NSArray (or NSSet) containing NSDictionaries with the keypaths that should be compared, and then enumerating through that. Something like this:

#define keyOriginal = "original"
#define keyComparisonItem1 = "comparisonItem1"
#define keyComparisonItem2 = "comparisonItem2"

    NSMutableArray * evaluations = [NSMutableArray array];

    [evaluations addObject:@{keyOriginal : @"idag3_orig", keyComparisonItem1 : @"idag2", keyComparisonItem2 : @"idag3"}];
    [evaluations addObject:@{keyOriginal : @"igar3_orig", keyComparisonItem1 : @"igar2", keyComparisonItem2 : @"igar3"}];
        // and so on...

    for (NSDictionary * evaluation in evaluations) {

        if ([statisticsObject valueForKey:keyOriginal] != 0) {
            tmp = (int)[statisticsObject valueForKey:keyOriginal];
        } else {

            tmp = [self compareValue:[statisticsObject valueForKey:keyComparisonItem1]
                           withValue:[statisticsObject valueForKey:keyComparisonItem2]];
        }

        [self formatText:[statisticsView valueForKey:keyOriginal]];

    }