Q21 - Bicycle safety expert

[SJK493]

The two-part question was difficult, and I think I have trouble with questions involving percentages. I read the evidence given by the bicycle safety expert that the bicyclist was riding on the left in 15, 17, and 25 percent of the cases. But I didn’t see how this leads to the argument that bicycling on the left half of the road is much more likely to lead to collisions with automobiles than is bicycling on the right since no information was provided relating to bicycling on the right (hard to put into context). But I didn’t know how this leads to the answer choice (B) the statistics it cites do not include the percentage of bicycling that took place on the left. I am completely lost, so do you mind walking me through the stimulus?

[ohthatpatrick]

Think of it as a binary: you're biking on the left or on the right.

Let's look at these three studies:
study 1: 15% of the bike-auto collisions were with riders on the left.
so ... 85% of the collisions were with riders on the right

study 2: 17% of the collisions had riders on the left,
so 83% had riders on the right.

study 3: 25% left side collisions / 75% right side collisions

Why on Earth would the author look at these and conclude that "Biking on the left is MORE LIKELY to get you in a collision?"

In all three of those studies, collisions with right side riders were at least four times as common.

The only way this argument could make sense is if the number of riders on the left is WAY smaller than the number of the riders on the right.

If only 10% of RIDERS go on the left side,
but 15-25% of COLLISIONS involve riders on the left side
then the left side looks more dangerous.

(If which side you rode on made no difference, then we'd expect that if 10% of riders are on the left side then 10% of collisions will be on the left side. Seeing any statistic HIGHER than the proportion of riders on the left makes it seem like the left is MORE dangerous, whereas seeing a statistic that was lower than 10% would have made the left seem SAFER than the right side)

If people were just as likely to bike on the left as on the right, then the author's three studies work totally AGAINST her conclusion. In order for us to make sense of whether this expert is saying something illogical or legit, we need to know about the underlying % of riders on the left side.

If the % of riders on the left side is lower than 15-25%, then the author's got a good case.
If the % is higher than 15-25%, then author's got it completely backwards.

That's why we can't evaluate the argument until we get (B).

Hope this helps.