Outliers using Standard Deviation

The Flint, Michigan Water Crisis

In 2015, Flint, Michigan made headlines for a water crisis when extremely high lead levels were detected in the city's water supply. Listen to the following National Public Radio interview with Dr. Mona Hanna-Attisha, the pediatrician who discovered the elevated levels of lead in children in Flint.

Contaminants in the Water

The water in Flint contained several contaminants throughout the crisis. The first contaminant detected was E. Coli from the Flint River. Byproducts from disinfectants used to eliminate the E. Coli caused more people to get sick. Then, the corrosive nature of the water caused the lead water system pipes to erode.

The Environmental Protection Agency (EPA) sets standards to regulate the amount of contaminants in public drinking water. The following table outlines the contaminants discovered in Flint water. The Maximum Contaminant Level Goal (MCLG) is the amount below which there are no expected health risks. The Maximum Contaminant Level (MCL) is the highest legal amount allowed in drinking water. 

✔ Discussion 1
​Discuss any observations you have about the water contaminant information. 

Finding Outliers with Standard Deviation

✔ Notes 1

A data point is considered an outlier of the normal distribution if it is more than two standard deviations away from the mean.

Be a Water Quality Chemist!

✔ Figure 1
Suppose the national average for positive coliform samples is 2% of total samples, with a standard deviation of .5%. Draw the normal curve for this distribution.

✔ Analysis 1
Your town's most recent water survey resulted in 4% of samples testing positive for coliforms. Is your town considered an outlier?

✔ Figure 2
To attempt to lower your water supply's coliform rates, the water sanitation department decided to add more chlorine to the supply. Two weeks later, you take a survey of 200 samples and find that the mean chlorine level is 2.87 mg/L with a standard deviation of .35 mg/L. Draw the normal curve for this distribution.

✔ Analysis 2
During further analysis of the data, you discover 10 water samples that are considered outliers but are not more than 3 standard deviations from the mean. Do you have to take action for these 10 homes because you exceeded the MCL for chlorine? Why or why not?

✔ Analysis 3
Because your mean chlorine level was approaching the MCL, you decide to run additional tests for trihalomethanes, a dangerous byproduct of chlorine. The following table summarizes your results. Run 1-Variable Stats on the data.
Identify any outliers in the dataset. Do you need to take action at any sites because you exceeded the MCL for trihalomethanes? Would you report that your town achieved the MCLG?

FlintWaterStudy

FlintWaterStudy, the independent research team from Virginia Tech that is working as part of a larger coalition to help resolve the uncertainties associated with the drinking water crisis in Flint, MI, will answer your questions. Ask us anything!