The enrollment forecast you present to sponsors is either a confidence builder or a credibility risk. Sites that consistently forecast enrollment timelines accurately — and meet those timelines — receive preferential study allocations. Sites that repeatedly revise forecasts downward lose study opportunities regardless of their clinical quality. Building an enrollment forecast sponsors trust requires four specific data inputs and a calculation methodology that accounts for real-world conversion variability. This guide walks through the exact calculation.
Why Most Enrollment Forecasts Are Wrong
Most sites forecast enrollment by dividing target enrollment by available weeks: “We need 40 patients in 24 weeks, so 1.67 per week.” This ignores four critical variables: the time lag from inquiry to enrollment (typically 3–6 weeks from first inquiry to enrolled patient), the screen failure rate (which determines how many screened patients are needed to produce one enrolled patient), the inquiry-to-screen conversion rate, and the current pipeline state (how many patients are already in process at each funnel stage).
The Four-Input Enrollment Forecast Model
Input 1: Current enrollment velocity. How many patients are you enrolling per week right now, based on the past 8 weeks of data? This is your baseline rate before any campaign changes.
Input 2: Pipeline conversion rates. What percentage of inquiries convert to a screen? What percentage of screens enroll? These rates, applied to your current pipeline, predict how many of your in-progress patients will ultimately enroll.
Input 3: Screen failure rate by criterion. If your screen failure rate is 45%, you need to screen 1.82 patients for every enrolled patient. Higher failure rates require higher inquiry volume for the same enrollment target.
Input 4: Current pipeline inventory by stage. How many patients are currently in each funnel stage — inquired but not pre-screened, pre-screened and pending visit, screened and pending enrollment decision? Apply the conversion rates to this pipeline to calculate projected enrollments from existing leads.
The Forecast Calculation
Remaining enrollment target = Total protocol target − Currently enrolled
Patients in screen pipeline × screen-to-enroll rate = Projected pipeline enrollments
Remaining target after pipeline = Remaining target − Projected pipeline enrollments
Required new inquiries = Remaining target after pipeline ÷ (inquiry-to-screen rate × screen-to-enroll rate)
Weeks to generate required inquiries = Required inquiries ÷ current weekly inquiry rate
Total weeks to completion = Pipeline processing time + new inquiry generation time
This calculation produces a date-specific enrollment completion forecast based on actual data — not a target divided by available time.
How to Present the Forecast to Sponsors
Present three scenarios: best case (current rates maintained), most likely (current rates with 15% variance buffer), and conservative (20% rate reduction). Show the data inputs for each scenario. Sponsors distrust single-point forecasts because they appear to ignore variability. Three scenarios with visible inputs demonstrate analytical rigor and allow sponsors to understand what conditions would affect the timeline.
48-Hour Action List
- Hour 1: Pull your last 8 weeks of enrollment data. Calculate your actual weekly enrollment velocity. Compare to your protocol-required rate. Calculate how many weeks ahead or behind pace you are.
- Hour 2: Calculate your current pipeline conversion rates from your tracking spreadsheet: inquiry-to-screen rate, screen-to-enroll rate, screen failure rate. These are the inputs for the forecast model above.
- Hour 3: Run the forecast calculation for your current study. Build three scenarios. Document the data inputs for each so the methodology is transparent to sponsors.
- Day 2: Present the three-scenario forecast in your next sponsor check-in. Alongside the forecast, include the one or two actions you are taking to maintain or accelerate the most likely scenario. Forecast + action plan is the combination that builds sponsor confidence in your site management.