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# S1 Normal Dist. Quick question watch

1. Hey guys,

just wondering for the normal distribution. How do I know which table to use since there are 2 tables. One for percentages and the other for the normal distribution. I lost a substantial ammount of marks because I used the normal distribution table instead of the percentage one
2. (Original post by Alex Dom)
Hey guys,

just wondering for the normal distribution. How do I know which table to use since there are 2 tables. One for percentages and the other for the normal distribution. I lost a substantial ammount of marks because I used the normal distribution table instead of the percentage one
Huh? One table gives you the value of phi(x), I presume, and one the value of phi^-1(x). (I think this is what you're talking about.) If you're looking for a probability, then you need to use the table that gives you a probability (which is the phi table); otherwise, the table that gives you not-a-probability (that is, the inverse phi table, which gives a number of standard deviations).

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