The Environmental Protection Agency (EPA) advocates a maximum arsenic level in water of 10 micrograms per liter. Below are results of EPA tests on randomly chosen wells in a suburban Michigan county. Research question: Is the mean arsenic level affected by well depth Arsenic and/or age of well?

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Arsenic Level in Wells (micrograms per liter) Age of Well (years) Well Depth Under 10 10 to 19 6.1 5.8 20 and Over Shallow 5.4 4.3 6.1 3.4 3.7 4.3 2.4 2.9 6.8 5.4 5.7 4.5 5.5 4.6 3.9 2.9 4.0 Medium 3.7 4.4 3.8 2.7 3.4 Deep