Clinical trial recruitment rate calculator and site selection
Every enrollment plan rests on one number: how many patients each clinical site needs to randomize each month. That number is your clinical trial recruitment rate, the number of patients randomized per clinical site, per month. Set it too high and the plan is behind before the first patient consents. Set it without accounting for slow starts and non-enrolling clinical sites, and the gap shows up months later, when it's harder to fix.
Use the formula and calculator below to set a realistic target, then use the site selection checklist to choose clinical sites that can meet it.
How to calculate your clinical trial recruitment rate
(total randomizations needed ÷ number of clinical sites) ÷ months of enrollment = randomized patients per clinical site, per month
For example, if a clinical trial needs 20 randomizations from 2 clinical sites over 5 months:
(20 ÷ 2) ÷ 5 = 2 randomized patients per clinical site, per month
The formula also works in reverse. To estimate how many randomizations your current plan can deliver, multiply the number of clinical sites by the expected randomizations per clinical site per month, then by the number of months of enrollment.
How does your target compare to published benchmarks?
A review published in BMJ Open analyzed 151 randomized controlled trials funded by the UK's National Institute for Health Research. It found:
- The median recruitment rate was 0.92 participants per clinical site per month.
- Only 56% of the clinical trials reached their final recruitment target.
The authors' advice to investigators was not to be overly optimistic about recruitment projections.
These were publicly funded UK clinical trials, so rates in commercially sponsored clinical trials will differ. But if your plan assumes each clinical site will randomize several patients every month from day one, it's worth checking that assumption against data from your condition area before you commit to a timeline.
How to adjust your recruitment rate for real-world delays
Account for ramp-up time
When a clinical trial first launches, it can take several weeks for a patient to move through the process:
- an online screener
- follow-up questions by phone
- a screening visit at the clinical site
- consent and randomization
Clinical sites also rarely open on the same day. The BMJ Open authors note that staggered clinical site openings mean most clinical sites don't recruit for the full enrollment period.
If you expect the first month to produce few or no randomizations, take it out of the calculation:
(20 ÷ 2) ÷ 4 = 2.5 randomized patients per clinical site, per month
That one adjustment raises each clinical site's monthly target by 25%.
Plan for clinical sites that don't enroll
An often-cited analysis from the Tufts Center for the Study of Drug Development (Tufts CSDD) found that 11% of clinical sites in multicenter global clinical trials failed to enroll a single patient.
Here's what that does to the math. Take a clinical trial that needs 100 randomizations from 10 clinical sites over 5 months. If one of those clinical sites enrolls no one:
- All 10 clinical sites enrolling: (100 ÷ 10) ÷ 5 = 2.0 per clinical site, per month
- 9 clinical sites enrolling: (100 ÷ 9) ÷ 5 = 2.2 per clinical site, per month
The other nine clinical sites have to carry an extra 11% each.
Factor in seasonality
Monthly targets aren't equally achievable across the year. Patients may be less available for clinical site visits during the year-end holidays, for example, so plan for lighter months.
What slows clinical site recruitment?
Even with a realistic target, several factors can push randomization rates below plan:
- limited patient databases at the clinical site
- unresponsive patient contacts
- complex protocols with restrictive eligibility criteria
For a deeper look, read the most common clinical trial recruitment challenges and how to beat them.
How to select clinical sites that can meet your recruitment target
Clinical site selection is one of the best chances to prevent delays before enrollment begins. If you're using a site selection questionnaire, include these five points.
- Plan patient recruitment before selecting clinical sites.
Sponsors sometimes wait to plan patient recruitment until every clinical site has been chosen. But clinical sites that are inconvenient for patients, or already saturated with competing clinical trials, are harder to fill. Ask each candidate clinical site about its patient recruitment methods and recent enrollment numbers. Clinical sites that maintain a patient database or work with patient recruitment companies may have additional channels for identifying potential participants.
- Choose clinical sites with experience in your condition area.
This matters most for clinical trials with complex protocols, such as Alzheimer's disease and oncology studies. A clinical site that already treats patients with the condition, or has recruited them before, needs less outreach support.
- Check the clinical site's technology.
A clinical site using current electronic data capture (EDC) and related technology can often enroll, randomize, and report data faster. Integration still matters: in the Tufts CSDD 2017 eClinical Landscape Study, 77% of sponsors and contract research organizations (CROs) reported difficulty loading data into their primary EDC system. Confirm too that the clinical site has the imaging equipment, devices, and other infrastructure your protocol requires.
- Ask about clinical site staff experience.
Go beyond asking which clinical trials the staff have worked on. Ask:
- how many clinical trials in your condition area the investigator and site coordinators have run in the past two to three years
- how many patients they randomized in those clinical trials, compared with the original target
- which site coordinators will be assigned to your clinical trial, and how many other clinical trials they're currently managing
- how much site coordinator turnover the clinical site has had recently
The answers show whether the staff can carry your enrollment target, not just whether they've run a clinical trial before.
- Understand clinical site startup cycle times.
Ask for cycle times in three categories:
- draft sponsor budget received to final budget
- institutional review board (IRB) submission to final approved materials
- fully executed contract to open for enrollment
A delay in any one of these pushes back the date your clinical site can randomize its first patient.
Frequently asked questions
How do you calculate a clinical trial recruitment rate?
Divide the total number of randomized patients needed by the number of clinical sites, then divide by the number of months in the enrollment period. For example, 20 randomizations from 2 clinical sites over 5 months equals 2 randomized patients per clinical site, per month.
What is a typical recruitment rate per clinical site?
It varies widely by condition and study design. A BMJ Open review of 151 publicly funded UK randomized controlled trials found a median of 0.92 participants per clinical site per month.
Why do clinical sites miss their recruitment targets?
Common causes include limited patient databases, unresponsive patient contacts, complex protocols, seasonal dips, and slow ramp-up after launch. An often-cited Tufts CSDD analysis found that 11% of clinical sites failed to enroll a single patient.
How do you adjust a recruitment rate for non-enrolling clinical sites?
Recalculate using only the clinical sites you expect to enroll. In a clinical trial needing 100 randomizations from 10 clinical sites over 5 months, losing one clinical site raises the target for each remaining clinical site from 2.0 to 2.2 per month.
