In 2003, the Integrated Approach to Community Development (IACD) organization introduced the chulli water purifier to homes in Bangladesh that had not previously had access to safe drinking water. It was designed to be made cheaply from local materials. The purifier uses sand filtration to remove organic particles and heat treatment to eliminate microbes from water.

Water samples from 15 different locations containing high levels of the bacterium E. coli were passed through the purifier at different flow rates and temperatures to test its effect on contaminated water. The shaded area of the graph below represents the recommended temperature and flow rate for using the purifier.

Evaluate the chulli purifier as a method of controlling microbial growth.

a. water purifier is effective in removing bacteria;
b. no bacteria in 12/15 test sites, regardless of temperature or flow rate;
c. flow rate is less important than temperature;
d. no information about how contaminated the water was before various treatments;
e. no information about how effective the water purifier is in removing other harmful bacteria/substances;

In e most were able to say that it was effective in removing bacteria, but few were able to spot that there was no information on how contaminated the water was beforehand or how effective it would be in removing other bacteria.