The analysis compared the earliest available estimated enrollment (the target) with the latest reported actual enrollment (the final count) in ClinicalTrials.gov records retained by DataLookout. The monitored population underrepresents trials that ended before 2024. The comparison uses the values as reported on the registry; participant counts were not independently verified.
Final enrollment compared with initial targets
Among trials that reported enrollment more than 10% below target, the median final count was 53% of target.
Nearly two thirds of Terminated trials finished below half of target, compared with one in ten Completed trials.
| Status | Trials | Under half<50% | Half to 90%50 to 90% | Within 10%90 to 110% | More than 10% over>110% |
|---|---|---|---|---|---|
| Completed | 19,045 | 10% | 20% | 52% | 18% |
| Terminated | 2,293 | 63% | 20% | 11% | 6% |
Just over a quarter of all trials reported a final count identical to the target; these make up most of the within-10% band. The registry records when the field changed from Estimated to Actual, but not how the Actual figure was determined.
Half of Phase 3 trials reported enrollment above target
Half of Phase 3 trials reported a final count above target, the highest share of any phase. Phase 3 trials also fell below half of target least often, at 12%.
Among Phase 3 trials, 61% of those led by Industry sponsors ended above target, compared with 26% of those led by sponsors the registry classes as Other, mostly universities and hospitals.
| Lead sponsor class | Trials | Above target | Under half |
|---|---|---|---|
| Industry | 1,188 | 61% | 8% |
| Other | 545 | 26% | 21% |
Larger trials fell below half of target more often
Of trials with a target under 20, 11% finished below half of target, compared with 21% of trials with a target of 1,000 or more. Among Completed trials alone the rise is steeper, from 5% to 19%.
Most records showed no revision to estimated enrollment
In 86% of trials, no change to estimated enrollment appeared in the available registry history before an actual count was reported. In Novo Nordisk's Phase 3 trial of semaglutide in early Alzheimer's disease, the estimate of 1,840 is the same in all 51 versions of the record from 2021 to 2026, and the final count was also 1,840.
The estimated enrollment for GSK's Phase 3 meningococcal B vaccine trial was reduced twice before actual enrollment was reported.
| Version date | Enrollment | Field type | Trial status |
|---|---|---|---|
| August 3, 2018 | 2,700 | Estimated | Recruiting |
| April 6, 2020 | 1,800 | Estimated | Recruiting |
| July 1, 2020 | 1,200 | Estimated | Recruiting |
| January 13, 2025 | 1,196 | Actual | Completed |
See each enrollment change on a trial record
Every DataLookout trial page lists the recorded changes to a trial's enrollment figure, with the old value, the new value and the date. Users can monitor selected trials, sponsors or conditions and receive an email digest when their records change.
Start freeData and methodology
Source and date. ClinicalTrials.gov record histories held by DataLookout. Data were frozen on October 24, 2026.
Cohort. DataLookout holds record histories for 165,405 of the trials it monitors. The analysis includes the 21,338 of those that are interventional studies (study type as recorded on ClinicalTrials.gov), whose latest version is marked Completed (19,045) or Terminated (2,293), whose latest Enrollment value is typed Actual, and whose earliest available version carries an Estimated enrollment above zero. Observational studies (4,693 trials that otherwise met the rules) are excluded. Withdrawn trials are excluded because zero enrollment is their definition. Trials with any other latest status are excluded.
Comparison. "Target" is the Estimated enrollment on the earliest available version of the record. "Final count" is the Actual enrollment on the latest version. The two are compared as a ratio. The analysis compares reported registry values and does not independently verify participant counts.
Bands. "Within 10%" means the final count was between 90% and 110% of target, inclusive. "Under half" means below 50%. "Half to 90%" and "More than 10% over" are the remaining two bands; the four are exclusive. "Above target" means above 100% and therefore overlaps the within-10% band.
No revision. Every Estimated value observed across the record's versions before the first Actual value was the same value: 18,278 of 21,338 trials (85.7%).
Coverage limits. The earliest available version is the original registration for 97.6% of the cohort; for the rest, a later version is the earliest available. The monitored set is built from trials that were active when DataLookout first saw them, so it under-represents trials that ended before 2024. Trials still active at the freeze date are excluded; the effect of this exclusion on the reported distributions was not measured. The results describe this cohort, not every trial registered on ClinicalTrials.gov.
Exact matches. Final count equal to target: 5,969 trials (28.0%); 58.7% of the within-10% band; 30.8% of Completed trials and 4.8% of Terminated trials; 20.5% of Industry-led trials and 31.0% of Other-class trials. Exact-match rates by sponsor class are not adjusted for trial size. The registry records the change of field type from Estimated to Actual but not how the Actual figure was determined.
Status split. Trials below half of target: 3,382, of which 1,944 (57.5%) are Completed. The four bands by status: Completed (19,045) 10.2% / 19.6% / 52.1% / 18.1%; Terminated (2,293) 62.7% / 20.3% / 11.0% / 6.0%.
Size of shortfalls and overruns. Among the 7,577 trials below 90% of target, the median final count was 53.3% of target and the lower quartile 30.0%. Among the 3,591 trials above 110%, the median was 135% of target. 6.2% of all trials exceeded 150% of target and 3.1% exceeded 200%.
Phase. The phase value on the latest record.
Sponsor class. The lead sponsor's class as recorded on ClinicalTrials.gov. Of the 545 Phase 3 trials with an Other-class lead sponsor, 397 have a lead sponsor whose name contains university, hospital, medical center or clinic, and most of the rest are hospital groups.
Size bands. Grouped by target.
Adjustment. All subgroup differences are descriptive and not adjusted for trial characteristics. Termination rates differ across subgroups (6% to 28% by phase), so the two tables below also give the below-half share among Completed trials alone; within Phase 3 it is 4% for Industry and 14% for Other.
Swipe sideways for the remaining columns.
| Phase (latest record) | Trials | Above target | Under half | Under half, Completed only |
|---|---|---|---|---|
| Early Phase 1 | 275 | 19% | 28% | 20% |
| Phase 1 | 2,081 | 26% | 19% | 10% |
| Phase 1/2 | 782 | 32% | 28% | 16% |
| Phase 2 | 2,866 | 36% | 21% | 11% |
| Phase 2/3 | 263 | 35% | 22% | 13% |
| Phase 3 | 1,819 | 50% | 12% | 7% |
| Phase 4 | 1,058 | 27% | 19% | 11% |
| Not Applicable | 12,194 | 26% | 13% | 10% |
Swipe sideways for the remaining columns.
| Target | Trials | Under half | Half to 90% | Within 10% | More than 10% over | Under half, Completed only |
|---|---|---|---|---|---|---|
| Under 20 | 1,691 | 11% | 19% | 48% | 22% | 5% |
| 20 to 49 | 5,723 | 14% | 20% | 48% | 18% | 8% |
| 50 to 99 | 5,159 | 15% | 19% | 50% | 16% | 9% |
| 100 to 299 | 5,393 | 18% | 20% | 47% | 15% | 12% |
| 300 to 999 | 2,429 | 18% | 19% | 47% | 15% | 13% |
| 1,000 and up | 943 | 21% | 22% | 40% | 17% | 19% |
Named trials. NCT04777396: Novo Nordisk A/S, Phase 3, semaglutide in people with early Alzheimer's disease, first posted March 2, 2021, marked Completed with a 2026 completion date; 51 versions, Estimated 1,840 throughout, Actual 1,840. NCT03621670: GlaxoSmithKline, Phase 3, meningococcal group B vaccine given with routine infant vaccines to healthy infants, first posted August 8, 2018, marked Completed with a 2024 completion date; 40 versions; Estimated 2,700 (version dated August 3, 2018), 1,800 (April 6, 2020), 1,200 (July 1, 2020); Actual 1,196 (January 13, 2025). Both reviewed on their DataLookout trial pages on October 24, 2026.
External source. Protocol Registration Data Element Definitions: "Enrollment", "Study Type", "Study Phase" and "Overall Recruitment Status" (ClinicalTrials.gov, accessed October 24, 2026). Figures were computed from a frozen internal copy of the data; the analysis script is not public.