Types of errors There are two types of errors that can be made when we test the null hypothesis against the alternative hypothesis.

- If we reject H0 in favor of H1, when H0 is true, then we say that we have made a first order error. we have committed an error of the first kind (also called type I). This means that we have concluded that there are effects, but in reality, there is no effect.

- If we do not reject H0, when H1 is true, then we say that we have made a two-step error. If we do not reject H0, when H1 is true, then we say that we have committed a second type of error (also called type II). This means that we have concluded that there is no effect, but in reality, there are effects.

Exercise 9.4

Studies have been conducted in a large metropolitan area to determine the concentration of carbon monoxide near major highways. There are concerns that the average concentration of carbon monoxide is above 100 parts per million. They captured the air in bags and used a spectrophotometer to determine the concentration of carbon monoxide in parts per million (ppm). Here are the results for 25 randomly selected times over a period of six months.