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DVar Method

You can use the DVar function to estimate variance across a set of values in a specified set of records. Use the DVar function in Visual Basic, a macro, a query expression, or a calculated control on a form or report.

Use the DVar function to evaluate variance across a population sample. For example, you could use the DVar function to calculate the variance across a set of students' test scores.

expression   Required. An expression that returns one of the objects in the Applies To list.

Expr  Required String. An expression that identifies the numeric field on which you want to find the variance. It can be a string expression identifying a field from a table or query, or it can be an expression that performs a calculation on data in that field. In expr, you can include the name field in a table, a control on a form, a constant, or a function. If expr includes a function, it can be either built-in or user-defined, but not another domain aggregate or SQL aggregate function. Any field included in expr must be a numeric field.

Domain  Required String. A string expression identifying the set of records that constitutes the domain. It can be a table name or a query name.

Criteria  Optional Variant. An optional string expression used to restrict the range of data on which the DVar function is performed. For example, criteria is often equivalent to the WHERE clause in an SQL expression, without the word WHERE. If criteria is omitted, the DVar function evaluates expr against the entire domain. Any field that is included in criteria must also be a field in domain; otherwise the DVar function returns a Null.

Remarks

If domain refers to fewer than two records or if fewer than two records satisfy criteria, the DVar function return a Null, indicating that a variance can't be calculated.

When you use the DVar function in a macro, module, query expression, or calculated control, you must construct the criteria argument carefully to ensure that it will be evaluated correctly.

You can use the DVar function to specify criteria in the Criteria row of a select query, in a calculated field expression in a query, or in the Update To row of an update query.

Note   You can use the DVar function or the Var function in a calculated field expression in a totals query. If you use the DVar function, values are calculated before data is grouped. If you use the Var function, the data is grouped before values in the field expression are evaluated.

Use the DVar function in a calculated control when you need to specify criteria to restrict the range of data on which the function is performed. For example, to display a variance for orders to be shipped to California, set the ControlSource property of a text box to the following expression:

=DVar("[Freight]", "Orders", "[ShipRegion] = 'CA'")

If you simply want to find the standard deviation across all records in domain, use the Var function.

Note   Unsaved changes to records in domain are not included when you use these functions. If you want the DVar function to be based on the changed values, you must first save the changes by clicking Save Record on the File menu, moving the focus to another record, or by using the Update method.

Example

The following example returns estimates of the variance for a population and a population sample for orders shipped to the United Kingdom. The domain is an Orders table. The criteria argument restricts the resulting set of records to those for which ShipCountry equals UK.

Dim dblX As Double, dblY As Double
' Sample estimate.
dblX = DVar("[Freight]", "Orders", "[ShipCountry] = 'UK'")
' Population estimate.
dblY = DVarP("[Freight]", "Orders", "[ShipCountry] = 'UK'")

The next example returns estimates by using a variable, strCountry, in the criteria argument. Note that single quotation marks (') are included in the string expression, so that when the strings are concatenated, the string literal UK will be enclosed in single quotation marks.

Dim strCountry As String, dblX As Double
strCountry = "UK"
dblX = DVar("[Freight]", "Orders", "[ShipCountry] = '" _
    & strCountry & "'")