March 18, 2013 7:00am by David Farrar

Landline vs Cellphone polling

The problem with calling cell phones doesn’t really lie in the cost of calls. For a polling company, calling a cell phone doesn’t cost that much more than calling a landline. The problem is the complexity and cost of employing dual sampling frames when the proportion of cell phone users without a landline is still very low. If the purpose of calling cell phones is to reduce non-coverage of likely voters, then you may actually need to ‘screen out’ those you call on cell phones who also have a landline (because they are already covered by the landline sample frame).

If we assume (hypothetically) that 6% of eligible voters have cell phones and no landline, that means that 94% of the people you call on a cell phone will not be eligible to take part (again, because they are already covered by the landline sample frame). This is where the cost would really begin to build up – all those interviewer hours required just to screen people out (eek!).

It is worth recalling the stat – you’d have to call almost 17 cell phones to find 1 person who doesn’t have a landline.

At the moment such a small proportion of New Zealanders have a cell phone with no landline that party support would need to be dramatically different among those people for this particular type of non-coverage to influence the poll results for party vote (eg, support for Labour among cell phone only voters may need to be twice what it is among landline voters for the party vote result to shift by more than, say, the margin of error).

Also a very useful point to recall when people raise the cellphone issue.

Let’s say National is at 48% amongst landline users and only 40% amongst cellphone only users. If you poll landline users only then you get National at 48%. If you poll both landline and cellphone only users you would have National at 47.5%. Not a huge difference.

Also worth noting almost all polling companies weight by age so any differences between landline and cellphone users which are due to different age profiles get compensated for anyway.

When the proportion of people with cell phones and no landline is considerably larger than it is today (like it is in some other countries), then it will definitely make sense to employ a dual sampling frame approach.

The results of the census will be interesting to see how the proportions have changed.