الدالتان DVar وDVarP

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You can use the DVar and DVarP functions to estimate variance across a set of values in a specified set of records (a مجال). Use the DVar and DVarP functions in a Visual Basic for Applications (VBA) module, a الماكرو, a query expression, or a عنصر تحكم محسوب on a form or report.

Use the DVarP function to evaluate variance across a population and 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.

بناء الجملة

DVar ( expr , domain [, criteria] )

DVarP ( expr , domain [, criteria] )

The DVar and DVarP functions have these arguments:

الوسيطة

الوصف

expr

Required. An expression that identifies the numeric field on which you want to find the variance. It can be a تعبير السلسلة 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. A string expression identifying the set of records that constitutes the domain. It can be a table name or a query name for a query that does not require a parameter.

criteria

Optional. A string expression used to restrict the range of data on which the DVar or DVarP 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 and DVarP functions evaluate expr against the entire domain. Any field that is included in criteria must also be a field in domain; otherwise the DVar and DVarP functions return a Null.


ملاحظات

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

Whether you use the DVar or DVarP 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 and DVarP 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.

ملاحظة: You can use the DVar and DVarP functions or the Var and VarP functions in a calculated field expression in a استعلام إجماليات. If you use the DVar or DVarP function, values are calculated before data is grouped. If you use the Var or VarP function, the data is grouped before values in the field expression are evaluated.

Use the DVar and DVarP functions 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 or VarP function.

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

مثال

ملاحظة: Examples that follow demonstrate the use of this function in a Visual Basic for Applications (VBA) module. For more information about working with VBA, select Developer Reference in the drop-down list next to Search and enter one or more terms in the search box.

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 ShipCountryRegion equals UK.

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

The next example returns estimates by using a variable, strCountryRegion, 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 strCountryRegion As String
Dim dblX As Double
strCountryRegion = "UK"
dblX = DVar("[Freight]", "Orders", _
"[ShipCountryRegion] = '" & strCountryRegion & "'")
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