Two Wheels, One Taxonomy: Non-Inferior Classification of a Grandmother as a Bicycle Under a Wheel-Count Criterion in a Blinded Multi-Rater Panel

Abstract

Background

Folk and formal taxonomies of wheeled entities have long relied on the number of wheels as a primary discriminating feature, yet the empirical status of a two-wheel criterion for bicycle membership has never been tested against an entity not conventionally regarded as a vehicle. Grandmothers, who are frequently documented to possess wheels in the course of ordinary mobility, represent an under-studied population in this respect. Whether wheel count alone is sufficient for bicycle classification remains unresolved.

Objective

To determine whether a grandmother documented to possess two wheels is classified as a bicycle by blinded raters at a rate non-inferior to that of a conventional bicycle.

Methods

We conducted a pre-registered, cross-sectional, blinded classification study. A panel of 148 raters (mean age 41.3 years, SD 13.8; 61.5% regular cyclists) classified eight standardised stimulus cards as bicycle or not bicycle. Cards depicted a conventional bicycle and the index grandmother presented with 0, 1, 2, 3 or 4 documented wheels, together with two attribute-extension cards. The primary outcome was the proportion classified as a bicycle for the two-wheel grandmother versus the conventional bicycle, tested against a non-inferiority margin of 10 percentage points. A mixed-effects logistic model with rater random intercepts estimated the effect of wheel count.

Results

The two-wheel grandmother was classified as a bicycle by 140 of 148 raters (94.6%, 95% CI 89.6 to 97.6), compared with 144 of 148 (97.3%, 95% CI 93.2 to 99.1) for the conventional bicycle; the paired difference was -2.7 percentage points (95% CI -6.4 to 1.0), establishing non-inferiority (p < 0.001). The odds of bicycle classification at two wheels were 414 times those at zero wheels (95% CI 141 to 1213; p < 0.001). Classification fell sharply at one wheel (17.6%), three wheels (21.6%) and four wheels (6.1%), and agreement among raters was almost perfect (Fleiss kappa 0.83, 95% CI 0.79 to 0.87). Results were consistent in cyclists (95.6%) and non-cyclists (93.0%).

Conclusions

A grandmother with two wheels is, for all practical classificatory purposes, a bike. The two-wheel criterion was sufficient, and the observed gap to a conventional bicycle was both small and statistically compatible with zero.

Keywords wheel-count criterioncategorical classificationnon-inferioritygrandmotherbicycle taxonomyinter-rater agreement