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# How to find z and t values for a confidence interval using a calculator instead of a table?

Previously, I've been using tables to find z and t values, and memorizing commonly used z-values like z(-1.96)=0.025 for a one-sided interval.

What about finding arbitrary values? Currently I'm looking for a t(-v)=0.025 t-value with 59 degrees of freedom. (Another 1-sided lower interval)

Is there a way to find this quickly on a calculator (TI-89)? My approach has been guessing t-values and punching them into the t_cdf function until I get a result close to the t-score I'm looking for.

Perfect. Thanks Salt, you've made my life a lot easier.

### 2 Answers

- 7 years agoFavorite Answer
2nd -> VARS(Distr) -> invNorm(for z) or invT (for T)

invNorm syntax

invNorm(confidence value)

for confidence value, you have to consider that the calculator defaults to having all of the area under the curve on the left of your point.

so if i wanted a 95% confidence interval i'd input

invNorm(.975)

invT syntax

invT(confidence value, df)

Source(s): AP Statistics 2012 - 2013 - Anonymous4 years ago
a) Apply paired t test (d.f. 6) b) t distribution c) Estimate what? Is it before or after or increase?