A popular way of doing forecasting for workforce planning today is the use of staffing ratios. Simply put, staffing ratios take a look at metrics such as revenue per employee and use these ratios to project workforce needs forward as revenue changes. In some cases, these ratios are used to look at both "direct" (revenue generating) headcount and administrative positions. While these ratios can be useful to give directional guidance, they often fall significantly short in developing appropriate workforce forecasts.
The problem with staffing ratios can be similar to those I described in the previous post on data eras. In any organization, there are "break points" where these staffing ratios become no longer valid. In some cases it is due to economies of scale achieved as an organization grows. In some cases it is due to achieving a critical mass in a certain area. In many cases, these ratios do not take into account potential strategies such as productivity increases, automation, or any number of strategies that change the fundamental ratio between growth and staffing. If an organization's metrics that drive their staffing ratios vary within a narrow range during the planning period, then staffing ratios may actually work just fine. But when looking at multiple scenarios, "stretch goals", etc., they often fall short.
Personally I often use staffing ratios to get "quick and dirty" trend lines or directions on staffing needs, but very rarely do I rely on them to make real projections.
Organizations can sometimes use staffing ratios effectively if they do additional analysis. In some cases, enough data exists that statistical analysis can reveal how staffing ratios change over time. The analysis here is similar to what I discussed in the post on data eras. But this additional analysis will still generally be limited to the types of scenarios that the business has seen in the past. And it still won't account for the factors such as productivity mentioned above.