Viewing a single comment thread. View all comments

[deleted] t1_ix80nf2 wrote

Reply to comment by seamustheseagull in A 20 mph speed limit intervention implemented at city centre scale had little impact on short- or long-term outcomes for road traffic collisions, casualties and speed. by _DeanRiding

The data does not suggest otherwise. As you pointed out the sample size is way too low to draw any reasonable conclusion. So i don't know why in your next sentence you completely ignore that and continue on as if it does.

If an intersection only has 3 incidents per year on average and then after the study there are 2 incidents, it doesn't mean we can conclude that there was a reduction because of the speed limit.

4

seamustheseagull t1_ix87oe4 wrote

No, but it may warrant further consideration. Certainly not conclusive enough to say, "no change was noted".

It's a common false-negative problem. "The differences were statistically insignificant" is only valid when you have a reasonable sample size. While technically it is correct to say, it implies a conclusion where none can be drawn.

2

[deleted] t1_ix92xnk wrote

>it implies a conclusion where none can be drawn.

No effect is literally a conclusion that can be drawn.

5

fatsynatsy t1_ixbw3n2 wrote

inappropriately drawn... as the sample size is too small to find a small effect with statiscal significance.

1

[deleted] t1_ixcpk6h wrote

Because the amount of accidents in the first place is small. So reducing speed limit isint going to have an effect since the effect was so small in the first place.

1

fatsynatsy t1_ixgrdio wrote

I see your point, but I would argue that no effect is still different from an effect which is functionally insignificant or outweighed by the inconvenience associated with a reduced speed limit.

1