Question

When do you use normalpdf, normalcdf, and invnorm?

Answer 1

normalpdf is a function that returns the probability of a random variable and a single value. You would use this function in order to return the y coordinate on a normal curve. normalcdf is a cumulative distribution function that returns the cumulative probability of the random variable x. invnorm is the inverse normal probability distribution function, it gives the value of x in a probability that is on the left side of the x value.

Answer 2

1: normalpdf     pdf = Probability Density Function
This function returns the probability of a single value of the random variable x.  Use this to graph a normal curve.  Using this function returns the y-coordinates of the normal curve.     
      Syntax:   normalpdf (x, mean, standard deviation)

#2: normalcdf    cdf = Cumulative Distribution Function
This function returns the cumulative probability from zero up to some input value of the random variable x.  Technically, it returns the percentage of area under a continuous distribution curve from negative infinity to the x.  You can, however, set the lower bound.
      Syntax:  normalcdf (lower bound, upper bound, mean, standard deviation)

#3: invNorm(     inv = Inverse Normal Probability Distribution Function
This function returns the x-value given the probability region to the left of the x-value.  
(0 < area < 1 must be true.)  The inverse normal probability distribution function will find the precise value at a given percent based upon the mean and standard deviation.
      Syntax:  invNorm (probability, mean, standard deviation)