Think about drawing a random sample only from cell phone exchanges. Discuss the advantages and disadvantages of such a sampling method compared with surveying randomly generated telephone numbers from non-cell phone exchanges. Do you think these advantages and disadvantages have changed over time? How do you expect they'll change in the future?

Consider the case of drawing a random sample only from cell phone exchanges instead of surveying randomly generated telephone numbers from non-cell phone exchanges.

With the passing of time, more and more people are leaning towards owning a cell phone instead of a land line.

The proportion of people in the population who use cell phones is gradually increasing when compared to non-cell phone users

Major fraction of the population now consists of cell phone users

In future there will be sharp increase in this fraction

So it will be advantageous to draw a random sample only from cell phone exchanges instead of surveying randomly generated telephone numbers from non-cell phone exchanges. It improves the chance of including more people (people who do not stay at homes during the time of survey will be contacted only though cell phones which is impossible in the case of non-cell phone users)

But there are some disadvantages. Not every person has an equal chance. People without cell phones are excluded. Cell phone users could not be contacted many times due to many reasons (absence of network coverage, power shortage, or if they were not allowed to carry their phones to work places or to some highly secured places)