Posts from May 2012 (43)

Team Ratings for Week 13

Here are the team ratings prior to Week 13, along with the ratings at the start of the season. I have created a brief description of the method I use for predicting rugby games. Go to my Department home page to see this.
 

Performance So Far

So far there have been 80 matches played, 57 of which were correctly predicted, a success rate of 71.2%.

Here are the predictions for last week’s games.

Predictions for Week 13

Here are the predictions for Week 13. The prediction is my estimated points difference with a positive margin being a win to the home team, and a negative margin a win to the away team.

Thanks for all those who sent in a nomination last week for our Stat of the Week competition. Please enter this week’s competition too!

With such a variety of nominations, it was hard for us to choose a winner but we particularly liked the critique of Conservative Party leader Colin Craig’s use of statistics from a Durex and Marie Clare surveys which was a hot topic of discussion in the country this week as well. (For details, see here and here. As per our rules, the first nominee of a particular statistic is eligible to win.

Savannah Post was the first to nominate this:

Aside from the fact that the statistic was obviously out of date, this is a classic example of quoting a statistic without giving any other information, potentially misleading readers and/or viewers. Quoting the number of men the average New Zealand woman has supposedly slept with in isolation gives no concept of how this statistic compares to similar countries, how the results were collected, which countries were included etc.

A disappointing misuse of statistics to promote the somewhat outdated concept that any woman who uses contraception is automatically promiscuous.

…closely followed by Patricia de Guzman:

Mr Craig has made this wild claim that young NZ women are the most promiscuous in the world when all he’s citing his information from are surveys conducted by a condom manufacturer (obviously people buying condoms will be having more sex; why buy them if you’re not planning on using them?) and Marie Claire (known to have sex info thus women reading this magazine are likely to be having more sex anyway).

Long story short, the sample from those surveys are not a representative of the NZ population of women. We don’t want the world thinking NZ women are promiscuous when there’s no proper evidence!

A big congratulations to Savannah.

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

• Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday May 18 2012.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of May 12-18 2012 inclusive.
• Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.
(more…)

There were two Stat-of-the-Week nominations (from Patricia de Guzman and Samantha Post) for the Conservative Party’s use of the Durex Sex Survey to oppose providing subsidised contraception.  We’ve seen this survey number before on StatsChat and it was in the NZ media when the figures were released, back in 2007.

What’s really striking about many of  the stories is the focus specifically on promiscuity of women, when the number that the survey was claiming to estimate is the same for heterosexual men and heterosexual women as a simple matter of arithmetic. 

As is so often the case, the political position seems to be developed independently and statistics sought to drape around it.  Or as Andrew Lang put it  they “use statistics as a drunken man uses lamp-posts — for support rather than for illumination.”

If you’d like to comment on or debate any of this week’s Stat of the Week nominations, please do so below!

In the Stat of the Week competition, Nick Iversen points to a poll with a margin of error given as 3.9% from a sample of 1004, where the simple formula based on sample size would give 3.1%, and writes

The US poll uses 3.9%. I wonder if they are accounting for sampling bias as well as sampling error. Sounds to me like the US pollsters are doing a more sophisticated measure of error than NZ ones.

Following the link to the pollers methodology, we find that weighting is responsible.  The poll answers are reweighted to known population totals (eg, from the census) for age, sex,  and race/ethnicity, and also reweighted based on phone numbers per household.  They say

“Once weights are final, the effective, or weighted, sample size – rather than the actual
sample size – is used to compute the survey’s margin of sampling error.”

So, the 3.9% maximum margin of error doesn’t account for sampling bias in any direct way (which would be hard to impossible), but it corrects for the reweighting that is done to reduce sampling bias.  The more variable the resulting weights, the lower the effective sample size and the higher the maximum margin of error.

There’s some discussion on Simon Jackman’s blog (Australian political scientist now at Stanford) saying that serious US polling organisations usually do adjust the margin of error because of the reweighting, but that Australian ones don’t seem to. It looks like NZ ones don’t either.

In the past, newspapers would usually at least refer to their bogus polls of convenience samples as “unscientific”.  Now the warning word simply seems to be “online”.  The Herald’s headline “Mother’s Day off the cards for most: poll” is followed by the lead “Almost half of New Zealanders won’t be celebrating Mother’s Day today, according to a New Zealand Herald online reader poll”.

If the poll were meaningful, it would also matter that the question is badly designed: there are four answers, of which you have to pick exactly one to vote.  The options are a card, a meal, breakfast in bed, or nothing.  If you’re taking Mum skydiving, or sending flowers, or buying her chocolate, or a new chainsaw, how are you supposed to respond?

Team Ratings for Round 10

Here are the team ratings prior to Round 10, along with the ratings at the start of the season. I have created a brief description of the method I use for predicting rugby games. Go to my Department home page to see this.

Performance So Far

So far there have been 72 matches played, 42 of which were correctly predicted, a success rate of 58.33%.

Here are the predictions for last week’s games.

Predictions for Round 10

Here are the predictions for Round 10

Team Ratings for Week 12

Here are the team ratings prior to Week 12, along with the ratings at the start of the season. I have created a brief description of the method I use for predicting rugby games. Go to my Department home page to see this.
 

Performance So Far

So far there have been 73 matches played, 52 of which were correctly predicted, a success rate of 71.2%.

Here are the predictions for last week’s games.

Predictions for Week 12

Here are the predictions for Week 12. The prediction is my estimated points difference with a positive margin being a win to the home team, and a negative margin a win to the away team.

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

• Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday May 11 2012.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of May 5-11 2012 inclusive.
• Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.
(more…)