Measuring water quality Biochemical oxygen demand BOD measur

Measuring water quality

Biochemical oxygen demand (BOD) measure organic pollutants in water by measuring the amount of oxygen consumed by microorganism that break down these compounds. BOD is hard to measure accurately. Total organic carbon (TOC) is easy to measure, so it is common to measure TOC and use regression to predict BOD. A typical regression equation for water entering a municipal treatment plant is

BOD = -55.43 + 1.507 TOC Both BOD and TOC are measured in milligrams per liter of water.

Solution

BOD = -55.43 + 1.507 TOC Both BOD and TOC are measured in milligrams per liter of water.

What is the predicted BOD when TOC=0? Values of BOD less than 0 are impossible. Why do you think the prediction gives an impossible value?

predicted BOD when TOC=0,

BOD = -55.43 + 1.507*0 = -55.43

If Values of BOD less than 0 are impossible, then Check the range of the TOC whether it includes 0.

we should never use a regression model to make a prediction for a point that is outside the range of our data because the relationship between the variables might change in the outside range.

the prediction gives an impossible value when the model does not fully or correctly capture the relationship between BOD and TOC.

Sometimes constant in regression is meaningless. A regression model is the ability to understand how the response variable changes when you change the values of the predictor variable.

You can try transformation like LOG BOD and redo regression.