Summary
Workday offers several reporting options to analyze headcount trends. You can create trending custom reports to analyze trends in financial and worker data, including headcount and attrition. To enable this, you need to activate "Worker Trending" in your tenant setup and run the "Create Worker Trending Data" task.
The "Trended Workers" data source is commonly used for historical headcount analysis, providing up to 36 months of data. Workday also provides a "Headcount and FTE by Month" report that can be displayed on the Management Reporting Dashboard.
Other data sources like "All Active and Terminated Workers" can be used for headcount as of a specific date, though historical monthly data would require running the report multiple times. "Workers by Company for Financial Reporting" is another option, especially if "Trended Workers" is not accessible to certain user groups.
You can create various report types, including matrix reports for headcount by company and composite reports to combine data from multiple sub-reports or show year-to-year trends. For specific scenarios, such as historical headcount for managers or average headcount calculations, calculated fields and careful report design may be necessary. Ensure proper security settings are in place for relevant domains like "Trended Worker Data" to allow access to this information.
Retrieved From Workday Documentation
From Workday
To analyze headcount trends, you can create trending custom reports 
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2. Workday allows you to create a trending custom report to analyze trends in financial and worker data, such as headcount and attrition 
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Here are the steps and options related to trending reports for headcount analysis:
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Enable Tenant for Worker Trending Data 
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- Access the Edit Tenant Setup – Reporting and Analytics task.
- Select the Enable Worker Trending check box in the Worker Trending section.
- To view the status of this change, access the Status tab on the Maintain Trended Workers task.
- Edit permissions for the Trended Worker Data domain in the Staffing functional area using the Domain Security Policies for Functional Area report.
- Set Up the Trended Workers Report Data Source.
- Run the Create Worker Trending Data task whenever you make changes to configurations on the Maintain Trended Workers task. Workday recommends running this task during non-peak hours

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Create Trending Reports 
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- You can create a custom trending report to analyze trends in financial and worker data, such as headcount and attrition

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- These reports can also be used as subreports in composite reporting

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- Security for creating trending reports requires the Custom Report Creation and Manage: All Custom Reports domains in the System functional area

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Management Reporting Dashboard 
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- The Management Reporting Dashboard is a configurable workspace that provides managers with visual insight into workforce operations

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- You can use the Maintain Dashboards task to configure the dashboard and select up to 6 reports to display

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- One of the reports Workday provides for display on the dashboard as a worklet is "Headcount and FTE by Month," which displays headcount and FTE by month, secured by the Trended Worker Data domain in the Staffing functional area

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From Customers
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Trending Reports for Headcount Analysis 
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- Start with trending reports for headcount analysis. There is extensive documentation available for setting up your tenant for these reports.

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- Trending reports can be used to show headcount numbers by month over a period, such as the past 2 years.

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- A simple trending report can be created using the "Trending Worker" Datasource, with row grouping by month.

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- The "Headcount and FTE by Month" delivered report uses the "Trended Workers" business object and is set up for calendar year and month trending.

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- Trended Workers data source provides a rolling 36 months of history.

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Data Sources for Headcount Reports
- Trended Workers: This is a common data source for headcount reports, especially for trending over time.

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- It is recommended for getting headcount as of a specific date.

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- The "Trended Workers for all Organizations" RDS filter is available, but access needs to be enabled for the "Trended Worker Data" domain.

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- Worker Trending by Company/Organizations is being looked at as an RDS filter on the Worker Trending object.

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- All Active and Terminated Workers: This data source can be used for headcount reports.

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- For headcount as of a specific date, prompt the report for "As Of Effective Date".

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- Using this data source for historical monthly data would require running the report multiple times for different effective dates and consolidating the output.

5.2.1.1.1
- Workers by Company for Financial Reporting: This data source can be used for headcount reports, especially if the "Trended Workers" data source is not accessible to certain user groups like Finance.

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Creating Headcount Reports
- Matrix Reports: These can be used for headcount by company, using "Trended Worker" or "All Active and Terminated Workers" as the data source.

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- Set Row Grouping to "Company" and Summarization to "Count".

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- Filter for "Active" workers and any other desired criteria.

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- Composite Reports: These can be used to combine data from multiple sub-reports, for example, to show year-to-year trends.

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- A composite report can be used to achieve monthly headcount by specifying the effective date for each column, though it requires monthly maintenance.

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- Headcount by Supervisory Organization: A composite report exists for this, which can be modified to look at calendar year instead of calendar month.

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Addressing Specific Headcount Scenarios
- Historical Headcount for Managers: If a manager runs a trended worker headcount report, and a worker or organization moves into their hierarchy, the report might incorrectly show the moved workers in the manager's historical headcount for periods before the move.

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- This behavior might be related to "assign superior" actions not being captured correctly in trended worker reports.

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- A potential workaround involves using a calculated field to extract the record date for row/column grouping and summarizing by 'Ending Headcount'.

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- To show headcount for the population a manager supported in a given month (not just currently), calculated fields might be needed, but this can impact performance.

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- Headcount Targets: If Headcount Plans (HCM) are implemented, the "Headcount Plans to Actuals" report can be used.

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- If not implemented, targets might be stored in Prism or custom fields.

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- The "Headcount Plan Structure" (do not use) is being deprecated, with Headcount Plans (HCM) as the replacement.

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- Average Headcount: For calculating average headcount per financial year in a trending report, the average can be based on opening and ending headcount in the period.

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- For quarterly attrition, calculating average ending headcount for a quarter (e.g., sum of monthly ending headcounts divided by 3) can be challenging with current summarization options.

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Troubleshooting and Considerations
- Discovery Board Drill-by Issues: When using a Discovery Board with trended worker data for quarterly reporting, drill-by values might not match visualization values, potentially summing monthly ending headcounts instead of taking the quarter's ending headcount. This should be reported to support.

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- Lookup Value as of Date vs. Effective Date: Reports using "Lookup Value as of Date" calculated fields might return different active employee counts compared to reports using "Effective Date" in runtime prompts, as the scope of the "as of date" in calculated fields can be limited.

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- Security: Ensure the necessary domain security policies, such as "Trended Worker Data" and "Worker Data: Organizations", are enabled for the relevant security groups (e.g., Accounting Manager) to access trended worker data.

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