STATISTICAL METHOD
Incidence & Prevalence Analysis
Statistical Consulting for Disease Frequency
We apply incidence and prevalence analysis within your study design, fixing the case definition and denominators, computing proportions and person-time rates correctly and reporting them with 95% confidence intervals in publication-ready form.
No Commitment Required
Strictly Confidential
In-House PhD-Level Statisticians
Fixed Project Fee
Same-Day Response
METHOD OVERVIEW
What Are Incidence and Prevalence?
Incidence and prevalence are the two basic frequency measures of epidemiology. Incidence counts new events in a population at risk: the incidence proportion divides new cases by the number at risk at the start, and the incidence rate divides them by the person-time at risk. Prevalence is the proportion of a population with the condition at one point in time, or during a period. In a steady state, prevalence is approximately incidence multiplied by the mean duration of the condition. Each measure is reported with a confidence interval, and comparison across populations usually needs standardization. OnlineSPSS, a specialist statistical consultancy for academic, clinical and professional research, provides incidence and prevalence analysis within your study design, with the relevant checks, estimates, uncertainty measures, interpretation and publication-ready reporting.
WHEN TO ENGAGE
When Is Frequency Analysis Appropriate?
Incidence and prevalence fit questions about how common a condition is or how often new cases arise, and these are the research situations in which researchers most often ask us to compute or confirm them.
Cohort Follow-Up and Person-Time Participants enter and leave follow-up at different times, so events must be related to the person-time at risk and not to a simple head count.
Prevalence From a Survey Sample A cross-sectional or population survey estimates the proportion with a condition, and clustering, stratification and weights must enter the confidence interval.
Comparing Populations or Periods Rates differ between regions, years or age structures, and a fair comparison needs rate ratios with intervals and, usually, age standardization.
Reviewing a Frequency Analysis A prevalence was reported as an incidence, or the denominator was unclear. Independent Statistical Review rechecks the numerators, denominators and intervals.
ANALYTICAL APPROACH
How We Apply Incidence and Prevalence
Within the family of Clinical, Survival & Epidemiological, every frequency measure is defined by its numerator, denominator and time frame before it is computed.
Denominator and Case Definition Before any rate is computed we fix the case definition, the population at risk and the risk period, since the same events give different rates over different denominators.
Proportions Versus Rates We keep incidence proportions, incidence rates and prevalence apart, because presenting a cumulative proportion as if it were a person-time rate is a frequent reporting error.
Intervals and Survey Design Exact Poisson or Wilson intervals suit simple counts. Clustered or weighted surveys need design-based variances, since ordinary binomial intervals are often too narrow.
Age Standardization Choices We use direct standardization when stratum-specific rates are stable, indirect when strata are sparse, and ask how many true cases the registry or record system may have missed.
DELIVERABLES
What You Receive
As part of our Quantitative Data Analysis service, the frequency measures are delivered with the checks and files needed to report and defend them.
Data & Variable Review Case definition, risk period, denominators, person-time construction, censoring and duplicate records are reviewed, and the population at risk is checked against the numerator.
Method-Specific Checks Numerator and denominator consistency, rate stability in small strata, overdispersion and survey design effects are examined, with sensitivity analyses for under-ascertainment.
Statistical Results Incidence proportions, rates and prevalence with 95% confidence intervals, rate ratios and differences, and crude and directly or indirectly standardized rates.
Publication-Ready Tables & Figures Rate tables, age-specific curves and trend figures, prepared in line with STROBE or RECORD reporting or the journal's requirements, ready to insert.
Interpretation Each measure is read as what it is, new events per person-time or existing cases at a time, with duration, ascertainment and denominators stated in the text.
Reproducible Analysis Files R scripts, Stata do-files, SAS or Python code reproduce every count, rate and interval, so the figures can be updated when new data arrive.
ALTERNATIVE METHODS
When Another Method May Be More Appropriate
Descriptive frequency measures answer how common and how often. These methods fit better when the aim is adjustment, competing events, pooling across studies or a change over time.
01
Poisson Regression
To compare rates while adjusting for covariates, Poisson regression with a person-time offset gives adjusted rate ratios, and a negative binomial model suits overdispersion.
02
Competing Risks Analysis
When death or another event can occur before the outcome, the risk over a period is a cumulative incidence that treats it as a competing event, not as censoring.
03
Proportion / Prevalence Meta-Analysis
To pool prevalence from several studies, a random-effects model for transformed proportions handles heterogeneity that a simple weighted average would hide.
04
Interrupted Time Series
To judge whether incidence changed after a policy or intervention, a segmented regression of the series separates a change in level from the existing trend.
CASE EXAMPLE
How We Work in Practice
An illustration of how frequency measures move from a prespecified definition to rates that can be reported and defended.
ILLUSTRATIVE EXAMPLE · OCCUPATIONAL INJURY SURVEILLANCE · UNITED STATES
An employer's occupational health service wanted to compare age-adjusted incidence of work-related back injury between two plants over a ten-year period, for its annual safety review.
The plants had different age structures, so crude rates would have confused workforce aging with risk. The plan fixed the case definition, person-year denominators from payroll hours and direct age standardization to a single standard population before any rate was computed, with gamma-based confidence intervals for the standardized rates. Late claim filing suggested under-ascertainment in the most recent year, so that year was labeled provisional and examined in a sensitivity analysis. A repeat injury in the same worker counted as a new case only after a stated recovery interval.
Outcome: The service received crude and age-standardized incidence rates with 95% confidence intervals, rate ratios between plants, the provisional-year sensitivity analysis, figures and the analysis scripts.
This example does not describe a real client or project. We never share details of actual client work, in line with the confidentiality we promise every researcher. This illustrative example is included to show the type, scope, and standard of work involved in this kind of analysis.
SOFTWARE
Statistical Software for Frequency Measures
Incidence rates, prevalence and standardization are run in the statistical software researchers already use, with syntax or scripts provided where applicable.
RESEARCH DISCIPLINES
Frequency Measures Across Research Fields
Incidence and prevalence analysis is used across research fields, each with its own case definitions, populations and reporting conventions.
A Defined Process for Your Analysis
We review your dataset, research question and design before any work begins, so the scope, deliverables, timeline and fixed project fee are clear from the outset.
01
Share Your Research Materials
Send your dataset, research question, study design and any existing analysis plan or output. We will let you know if anything else is needed to assess the project.
02
Receive Your Assessment
Your statistically trained Research Coordinator reviews the project and confirms the required analysis, deliverables, timeline and fixed project price before work begins.
03
Analysis & Delivery
Once you confirm the scope, an in-house statistician completes the agreed analysis and delivers the outputs, interpretation and reporting files.
RESEARCHER VOICE
What Researchers Say
Feedback from a researcher who needed the clustered structure of the data handled, not every row treated as independent.
“Our first pass at Incidence & Prevalence Analysis produced output, but I was not comfortable signing it off because of the clustered structure of the data rather than treating every row as independent. The analysis was rebuilt carefully and the interpretation was much clearer. The final reporting was much easier to use in the manuscript and in responding to reviewer comments.”
RESEARCH RESPONSIBILITY
Statistical Support Without Taking Over Your Research
We provide the statistical work for your frequency measures, and the study, its meaning and its authorship stay entirely with you.
Agreed Statistical Support We provide the agreed methodological, analytical and statistical reporting support for your incidence and prevalence analysis, within the scope confirmed before work begins.
No Intellectual Contribution OnlineSPSS claims no intellectual contribution to your research. Our work is statistical support, and authorship remains yours.
Your Research Decisions The case definition, the population of interest and your conclusions remain with you, and our statistical interpretation stays within what the counts, denominators and design support.
Confidential Handling Research data and project details are handled confidentially and are not published. An NDA is available where required, and can be arranged before sensitive materials are shared.
FREQUENTLY ASKED QUESTIONS
Questions About Incidence and Prevalence Analysis
Short answers to what researchers ask before reporting disease frequency, from rates and surveys to confidentiality and the assessment process.
What is the difference between incidence and prevalence?
Incidence counts new cases over a period among people at risk, whereas prevalence counts existing cases in a population at a point or during a period. Prevalence reflects both incidence and how long the condition lasts, so a chronic disease can have low incidence and high prevalence. Mixing the two, or comparing a proportion with a person-time rate, is a common reporting error.
Can I use a binomial confidence interval for survey data?
Only if the sample is a simple random sample. In a clustered, stratified or weighted survey, ordinary binomial intervals are usually too narrow because they ignore design effects. We use design-based variance estimates that reflect the sampling plan, and for small counts or proportions near zero we choose Wilson or exact methods and state which was used.
When is age standardization needed for comparing rates?
When the populations compared differ in age structure and the outcome depends on age, crude rates mix a difference in risk with a difference in age mix. Direct standardization applies stratum-specific rates to a standard population, and indirect standardization suits sparse strata. We choose the standard population openly, since it changes the absolute figures.
Can you review an analysis that has already been completed?
Yes, we can review a completed frequency analysis before submission or a response to reviewers. We check the case definition, the denominators and risk period, whether proportions and rates were kept apart, the interval method, any standardization and the handling of survey design, and confirm what stands or needs to change.
How do you protect my dataset and research confidentiality?
Your registry extracts, survey microdata and any protocol or analysis plan are handled confidentially, and project information is not published. An NDA is available where required and can be arranged before sensitive materials are shared. Read more about confidentiality and data security.
What happens when I request an assessment, and how long does it take?
You share your research question, design and relevant materials, and a statistically trained Research Coordinator reviews the request and responds the same day. The analysis, deliverables, timeline and one fixed project fee are then confirmed before any work begins, so the timeline follows from the agreed scope.
Still have a question? Find more answers in our full FAQ.
NEXT STEP
Need Support With
Incidence and Prevalence?
Share your research question, case definition and data. We assess which frequency measures and intervals fit your design and confirm the approach before any work begins.
No Commitment Required
Strictly Confidential
Fixed Project Fee
In-House PhD-Level Statisticians
Same-Day Response
Serving Researchers Since 2014