10.3.33.10 Specifying inputs for shorter time periods

STEM lets you enter sub-annual data if you want to – for example, to examine in detail the crucial early phases of a model – but does not require you to provide such detailed data for other periods. The new period input recognises anything from a year to a sub-annual period such as a quarter or month, through to a fully qualified date (which can be regarded as a one-day period). This new type replaces all the former year inputs, so we now have:

• periods for Interpolated Series
• an explicit Base Period for Exponential Growth and Floor and Multiplier inputs
• Base Period, Period A/B, Birth Period and Death Period for S-Curve inputs
• Calibration Period for Services and Resources.

The Model Start Date is entered as a period, which can be either a year (implying 1 Jan), or a fully qualified date if you want to model in financial years, e.g., from 5 Apr2001. For any other period input, all of the entries shown in Figure 1 below are valid.

In fact, the Editor is very forgiving, and will accept almost any date separator, e.g., space, hyphen, slash or any other punctuation. The day, month and year of a date are displayed in the order specified in your Windows International date settings; but months are always shown as short names rather than numbers to avoid ambiguity. Similarly, absolute calendar years are shown with four digits to avoid confusion with relative years, which are always prefixed with a ‘Y’.

Figure 1: Sample Period inputs

Non-standard years

There are some additional considerations for specifying annual and monthly data if you want to use a Model Start Date other than 1 Jan, e.g., when modelling financial years from 5 Apr 2001.

Four-digit year labels have a strong implicit association with the calendar year from 1 Jan to 30 Dec, and so it is common to use a hybrid label such as 2001/02 to refer to a non-standard year. However, this format is not ideal for data recognition, and also requires two numbers. Therefore, the identifier Y2001 is used as shorthand for 2001/02 (i.e., the year from 5 Apr 2001), consistent with the interpretation of relative years Yn (for integers n < 1000), which always run from the Model Start Date. This gives Yn a general (and machine-friendly) interpretation as ‘the model year starting in calendar year n ’.

Note: STEM displays hybrid year labels on outputs such as graphs and tables.

For completeness, if you specifically want to enter data for a relative calendar year, then you can use the prefix ‘C’, e.g., you can enter C1 to denote the calendar year (i.e., the period 1 Jan to 30 Dec) in which the model starts. Thus Cn has a general interpretation as ‘the calendar year in which model year n starts’.

Non-standard months

Labels for quarters, Q1 for example, are interpreted relative to either a model year or a calendar year. In contrast, month names are always interpreted in terms of calendar years. However, if you want to enter monthly data for non-standard years, you can use the labels Mn (for integers n in the range 1-12) to identify the n th period of thirty days in a year. So with a Model Start Date of 5 Apr 2001, the labels M1 and M12 identify the periods 5 Apr to 4 May 2001 and 5 Mar to 4 Apr 2002 respectively.

Note: If you enter labels such as Q5 or M13, these are automatically wrapped to the corresponding period in the next year, i.e., Q1 Y2 or M1 Y2, up to an arbitrary limit of 99, beyond which the input will be treated as an error.

Figure 2: Sample Period inputs for use with non-standard Years

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