Cruz has the backing of 28% of Republican voters nationwide, unseating Trump, who won the support of 26% in the latest NBC News/Wall Street Journal poll. But Cruz's 2-point edge is within the poll's margin of error, and it's not clear if the survey captures real movement in the race or is simply an outlier. The results are a major change from last month's NBC News/Wall Street Journal poll, when Trump held a 13-point lead over Cruz, 33% to 20%.
"I have never done will in the Wall Street Journal Poll. I think somebody at Wall Street Journal doesn't like me, but I never do well in the Wall Street Journal poll," Trump said. "So I don't know. They do these small samples and I don't know exactly what it represents."
Another Wall Street journal poll released Wednesday found registered voters to be divided on whether the Senate should vote this year on a replacement for late Supreme Court Justice Antonin Scalia. Forty-three percent believe the Senate should vote on a replacement this year rather than wait for President Barak Obama's term to end, versus 42% who oppose a vote.
The NBC New/Wall Street Journal pollsters contacted 800 registered voters for the question on the Supreme Court, with a margin of error of +/-3.5 percentage points, and 400 Republican primary voters for the Republican field, with a margin of error of 4.9 percentage points.
There are some good mathematical points to be made in this article.
- The first paragraph could lead to a discussion of what an outlier is.
- The second paragraph shows that candidate Trump should probably do some research on how polls and "these small samples" work.
- The final paragraph speaks to the concept of margin of error. Note that the margin of error was greater when fewer people were polled.
The margin of error is usually not well explained when used in news reports and is generally misunderstood. A poll result of 43% with a margin of error of 3.5% would imply a range of 39.5 to 46.5%. However, the true result isn't necessarily within that range. Pollsters tend to state that the true result would be in that range with a probability of 90%, 95%, or 99%. The article doesn't say what that probability is.
We can figure it out, though. With a sample size of n, a 99% level of confidence yields a range of 1.29/sqrt n. A 95% level is 0.98/sqrt n. A 90% level of confidence is 0.821/sqrt n.
Most polls use a 95% level of confidence and that, in fact is the case here. Note that 0.98/sqrt(800) = 3.46% and that 0.98/sqrt(400) = 4.9%, which agree with the values given in the article.
