My intution was that defenders convert penalties just as frequently as attacking players in World Cup shootouts. I figured that anyone at the World Cup was an extremely highly-skilled player and to be chosen to take a kick meant that one had already been vetted by the team's coaching staff.
Anyhow, penalty shootouts have only been a part of the World Cup since 1982 and all the data is available on wikipedia. All I had to do was use the positions from the match reports to get some conversion rates for each position. Simple enough.
For a few players, the distinction between the different positions of my choosing (Defender, Midfielder, Forward) were not exactly straightforward but I think it all comes out in the wash.
Anyhow, the results are below:
So it seems that forwards do convert penalties more frequently than other players by a non-negligible margin. Also interesting that defenders and midfielders seem to have converted penalties at effectively identical rates.
Once I had the data, it was trivial to rank countries that have participated in World Cup penalty shootouts by their conversion rates:
|Republic of Ireland||0.7||10|
And lastly, I wasn't expecting to really find anything here but here are the conversion rates for the different tournaments since shootouts have been used at the World Cup: