All from "The Football Handbook":
- Who calculates the extra time: the referee or an official off the pitch? - The referee determines the amount of stoppage time in a football match, based on the amount of playing time that has been lost in the half of the match.
- What does the referee (or official off the pitch) take into account when he adds time? Are there clear rules about what constitutes a stoppage that should not count towards the 45 minutes for the half? - See 1, the referee decides
- Why is the amount of extra time always a round minute (and if the referee/official has to round, does he round up or down)? - Once the referee decides on the appropriate time, he communicates this to the fourth official. The fourth official displays the time allowed, which is the minimum amount of minutes.
- Why does the game not end precisely on the rounded final minute, and what are the rules that govern when the referee blows the whistle once the rounded final minute has been reached? - See 3.
- Why do they not add time during the extra time period if there is a stoppage? - From what I'm seeing in the World Cup, I believe they do, at the 15 and 30 minute points.
You didn't ask, but I sense the question is in your mind: Is stoppage time really accurate? - The stoppage time gives rise to a fair amount of controversy in games. Mistakes are likely to occur as it is entirely the referee’s decision. Hence, the stoppage time is not always accurate but is usually a fair reflection of the game.
All that being said, in this World Cup more stoppage time seems to be added than in previous competitions. I think this is excellent, and should negate the efforts of time-wasters to run down the clock. I hope it will be more automated in the future, taking the decision out of the referee's hands, since the quality of the non-automated decisions that are down to the referee is absolutely abysmal compared to the automated VAR decisions (like offside, ball out of play, etc).
I shall take a step back to graciously allow someone else to answer Ronnie's question
The WP posted a correction to the article today I believe, essentially undermining the whole purpose of the article, though their retraction looked a bit odd to me.