Donegal have the meanest defence of any semi-final team since the back door was introduced, but face their toughest test to date in the shape of Dublin’s much-vaunted attack
AS Mayo discovered last Sunday, the past is not always a reliable indicator for the future, but it can't be ignored either.
Donegal's recent past -- meaning their five championship games this year -- will take them into Croke Park tomorrow with a defensive return which no All-Ireland semi-finalist can match since the change in the championship system 10 years ago when, apart from opening the 'back door' to provincial losers, quarter-finals were added to the schedule.
And as the semi-finalists since 2001 featured Kerry (10 times), Cork (seven), Dublin (five), Tyrone (four), Meath, Armagh, Mayo (three each), Derry (twice), Galway, Down, Wexford, Fermanagh, Kildare (all once) as well a previous Donegal team in 2003, it underlines the level of vigilance deployed by Jim McGuinness' watchmen.
Donegal have conceded the remarkably low average of nine points (1-42 in five outings, excluding extra-time against Kildare) per game. Kildare scored four points in extra-time but it would be unfair to treat that in the same way as normal time since fatigue is always a factor when players have to face an added 20 minutes.
When extra-time is omitted from Donegal's equation, their average giveaway is nine points, ranging from a high of 1-8 against Cavan to a low of 0-7 against Antrim. The Cavan score has a context too as both sides had a man sent off in the first quarter, creating extra space which may have accounted for a few scores.
Donegal's nine-point concession rate is five lower than Dublin and Kerry and two lower than Mayo, prior to last Sunday. It all changed for the worse for Mayo as Kerry hit them for 1-20, most of which came in the second half, a period in which the Connacht champions had been particularly secure in previous games.
Donegal have been tight in both halves but are about to be confronted by a Dublin team which has averaged 19 points per game in league and championship this year. Clearly then, something has to give when a strike force with an average 19-point return meets a defensive system which is programmed to give away around nine points.
Indeed, that's one of the more fascinating aspects of tomorrow's game. Dublin will, no doubt, have studied Donegal's defensive systems in detail in an attempt to figure out the combination to the locks. Mind you, Donegal use a fairly simple formula, with players energetically flooding space and frustrating the opposition into making errors.
There has been some criticism of the Donegal strategy, right from the opening day of the Ulster championship when Antrim manager Liam Bradley said that he wouldn't have paid in to watch the game.
However, McGuinness could counter by pointing out that his job is to get the best results possible from the players at his disposal and that the system they are operating maximises their prospects.
It's a fair point. Donegal had not won an Ulster championship game since 2007 until this year; they conceded 1-27 to Cork in the 2009 All-Ireland quarter-final and were hit for 2-14 by Armagh in last year's qualifiers.
Joe Kernan, a man with a well-deserved reputation for setting up tight defences, has been amazed by the transformation in Donegal since then.
"They are working harder for each other than any Donegal team I've even seen," Kernan said. "Jim has them playing to a system they know and trust and which has delivered the right results. It means thousands of happy Donegal people are heading for Croke Park convinced that there's an All-Ireland in this team.
"That's some change from last year when many of the Donegal players looked like lads who couldn't wait to be out of the championship the day they played Armagh. They had no appetite for it at all."
The tightening of the defensive screw was a gradual process in the Allianz League. Sligo, Derry and Laois (twice) all ran in quite high scores against Donegal but Tyrone, Kildare and Meath averaged only eight points each.
Still, there was no clear evidence as Donegal left Croke Park after beating Laois in the Division 2 final in late April that they would return for the championship with such a miserly defensive alignment.
Nominally, Frank McGlynn, Neil McGee, Karl Lacey, Anthony Thompson, Paddy McGrath and Kevin Cassidy are the Donegal defenders but, in reality, all 15 are part of the security detail.
It calls for an incredible work rate but the players -- who have willingly bought into the McGuinness plan -- haven't flinched all summer.
"You could see them growing in confidence with every game. They believe in themselves and what they're doing and so far it has worked," said Kernan.
Inevitably, Donegal's low giveaway rate shifts the focus on to the opposition and the question of how they rated in All-Ireland terms at the start of the season. Nobody reckoned Antrim or Cavan would make much of an impression, but improving Derry were seen as a threat while Tyrone and Kildare were regarded as genuine All-Ireland contenders.
In all cases, Donegal maintained a broadly similar defensive solidity but are now stepping up in class against Dublin, who produced one of the outstanding point-scoring performances of the season when hitting Tyrone for 0-22 in the quarter-final.
There are many who believe that not even Donegal's vaunted defensive banks can withstand the level of pressure that Bernard and Alan Brogan, Diarmuid Connolly and Co will build up, but then the Kildare and Wexford (up to the concession of an own goal) defences did well against Dublin.
That raises the possibility that it was Tyrone weaknesses rather than Dublin's super-power which defined the quarter-final.
Donegal's top security rating has certainly stood to them but it has come at the expense of high returns at the other end. With the exception of the game against Cavan, where they hit 2-14, their average score has been just under 13 points per game.
A 13-point total loses more All-Ireland semi-finals than it wins but then a 0-9 giveaway wins more than it loses.
Notice anything about those scores? They were precisely what came up in 1992 when Donegal last won an All-Ireland semi-final, beating Mayo 0-13 to 0-9.
Donegal would be more than happy with a repeat of that scoreline tomorrow.