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Alternative hypothesis: The 79% who were able to complete the trial had already selected for 'grit', or folks who wouldn't skip a day's work for a minor ailment. To check, instead of just comparing the test group to the control group you would need to compare the test group to itself, before and during the cold shower period.
It's built into the type system and checked statically at compile time. In rust, there can only be one 'owner' of a stored value. Ownership can be transferred, borrowed by multiple readers, or borrowed by a single writer. This ensures (at compile time) that there at any given time there is only ever one single writer OR multiple readers. Rust programs are guaranteed to be free of race conditions at compile-time. The above covers the vast majority of situations, but for cases where this is too restrictive their are other language constructs such as reference-counted types.
Another cool language from the past, 'nice', had a similar issue that sentenced it to obscurity. A totally ungooglable name :-(