Do you vary more than you differ from other people?
Most research about how people work describes people on average. Whether any of it can tell one particular person what to change is a different question, and mostly an unasked one. I write about that gap.
Evidence One statistic appeared to prove the gap is enormous. A 2018 paper in PNAS. The variation inside one person is roughly 8 times the variation between people. 7.85 to 1.
I wanted it to be true. I no longer think it is.
They took studies where people were measured over and over for weeks. Then each measurement was used twice. Once as a point inside one person’s record. Once as a stand-in for a whole person in a pooled group.
The second use is the problem. That between-person figure is not the spread of people. It is the spread of averages. Averages barely move, because smoothing out variation is the entire job of an average. So the comparison sets one person’s raw bounce against how much a set of averages differ. The first is always bigger. That is arithmetic, not psychology.
You can watch it happen. The spread of averages shrinks by roughly the square root of how many measurements went into each one. One of their samples had 78 observations. The square root of 78 is 8.83. The ratio they reported for that sample ran from 7.04 to 9.92.
The clearest sign sits in their own table. Depressed mood, rated 0 to 100. The reported spread between people was 2.77. That would mean everyone in the study has the same average mood within about 3 points out of 100. Nobody believes that about people. It is what you get when you measure how precise an average is and then call it the spread of a population.
Losing the number does not settle the question against me. The argument that group statistics do not describe individuals is mathematical and it stands on its own. The experience-sampling work stands too, in a narrower form than it usually gets quoted in. What died is one number, and it was the best-known one I had.
I set myself a rule. Go looking for the study that contradicts you before you publish. This one contradicted me and it was on my side.
Where this stops. Fisher, Medaglia and Jeronimus 2018, PNAS 115(27), open access at PMC6142277. The authors gesture at the mechanism themselves in their discussion. I have not seen a published paper making this criticism, so I am stating it as a reading of their own reported numbers rather than as an accepted correction.
What would change my mind. A response showing the between-person column is a spread of raw scores rather than of means.