The Iowa Department of Natural Resources (DNR) is responsible for environmental protec- tion in the state of Iowa. The DNR is conducting a study whose purpose is to determine the average concentration of the toxic mineral lead in the tap water of all Iowa homes built before the year 1920. There are more than 100,000 such homes.

The generally-accepted safe level of lead concentration in tap water is at most 15 micro- grams per deciliter (mg/dl). In contrast, lead concentrations that exceed (are greater than) 15 mg/dl are considered dangerous. Tap-water lead concentrations are known to be normally distributed. The DNR randomly selects 12 records from a database that includes tap-water lead concen- tration measurements for individual Iowa homes built in the years 1880-1950. Those records are shown below, including each home's tap-water lead concentration and build year:

describe the population?

describe the sample?

what variable?