... it doesn't exist.
One in four people will suffer a mental health problem/illness/issue at some point in their life/the past week/the past year (delete as appropriate) is a statistic seen all over any type of mental health media. It has never sat well with me. Mostly because I always think if an individual is unsure about their current state of mental health, it is easy to read that statistic and be reassured that 75 percent of people will never have a mental health problem... ever. And go ahead and place themselves in that 75 percent 'I'm going to be alright' bracket.
I'm not sure mental health problems work that way. For instance, if I were to say to you 1 in 4 people will suffer a physical health problem at some point in their life, you would look at me like a crazy person. It is quite clearly 100 percent unless anyone has figured out how to 'cure' mortality - or vampires exist, one of the two. We all get colds, and suffer from the various physical problems caused by ageing. There is a spectrum of severity here, but all are bannered underneath physical health nonetheless. Why are mental health problems treated any differently?