MORE INFORMATION
CHIINV(p, df) is the inverse function for CHIDIST(x, df).
For any particular x, CHIDIST(x, df) returns the probability that a
Chi-Square-distributed random variable with df degrees of freedom is greater
than or equal to x.
The CHIINV(p, df) function returns the value x
where CHIDIST(x, df) returns p. Therefore, CHIINV is evaluated by a search
process that returns the appropriate value of x by evaluating CHIDIST for
various candidate values of x until it finds a value of x where CHIDIST(x, df)
is "acceptably close" to p.
Syntax
Note In this example, p is a probability with 0 < p < 1 and df
>= 1 is the number of degrees of freedom. Because in practice df is an
integer; if a non-integer value is used, Excel will truncate it (round it down)
to an integer value.
Example Usage
To illustrate the CHIINV function, create a blank Excel worksheet,
copy the table below, select cell A1 in your blank Excel worksheet, and then
click
Paste on the
Edit menu so that the
entries in the table below fill cells A1:F21 in your worksheet.
| | actual | sales | | |
| 0 | 1 | 2 | 3 or
more | |
| before | 13 | 8 | 5 | 4 | =SUM(B3:E3) |
| during | 8 | 10 | 6 | 6 | =SUM(B4:E4) |
| =SUM(B3:B4) | =SUM(C3:C4) | =SUM(D3:D4) | =SUM(E3:E4) | =SUM(B5:E5) |
| | | | | |
| | expected | sales | | |
| 0 | 1 | 2 | 3 or
more | |
| before | =$F$3*B5/$F$5 | =$F$3*C5/$F$5 | =$F$3*D5/$F$5 | =$F$3*E5/$F$5 | |
| during | =$F$4*B5/$F$5 | =$F$4*C5/$F$5 | =$F$4*D5/$F$5 | =$F$4*E5/$F$5 | |
| | | | | |
| | (actual - expected)^2 | /
expected | | |
| 0 | 1 | 2 | 3 or
more | |
| before | =((B3-B9)^2)/B9 | =((C3-C9)^2)/C9 | =((D3-D9)^2)/D9 | =((E3-E9)^2)/E9 | |
| during | =((B4-B10)^2)/B10 | =((C4-C10)^2)/C10 | =((D4-D10)^2)/D10 | =((E4-E10)^2)/E10 | |
| | | | | |
| =SUM(B14:E15) | | | | | |
| =CHIDIST(A17,3) | | | | | |
| =CHITEST(B3:E4,B9:E10) | | | | | |
| =CHIINV(A18,3) | | | | | |
| =CHIINV(0.05,3) | | | | | |
Note After you paste this table into your new Excel worksheet, click
the
Paste Options button, and then click
Match
Destination Formatting. With the pasted range still selected, point to
Column on the
Format menu, and then click
AutoFit Selection.
To test the effectiveness of a
sale, a store records the number of deluxe freezers sold per day for 30 days
before the sale and for 30 days during the sale (see note 1). The data is in
cells B3:E4. The Chi-Square statistic is calculated by first finding the
expected numbers in each of these cells. These expected sales numbers are in
cells B9:E10. Cells B14:E15 show the quantities that must be summed to
calculate the Chi-Square statistic that is shown in cell A17. With r = 2 rows
and c = 4 columns in the data table, the number of degrees of freedom is (r -
1) * (c - 1) = 3. The CHIDIST value in cell A18 shows the probability of a
Chi-Square value higher than that in A17 under the null hypothesis that actual
sales and before or during are independent. CHITEST semi-automates the process
by requiring only B3:E4 and B9:E10 as inputs. It essentially deduces the number
of degrees of freedom and calculates the Chi-Square statistic and then returns
CHIDIST for that statistic and number of degrees of freedom. A20 shows the
inverse relationship between CHIDIST and CHIINV. Finally, A21 uses CHIINV to
find the cutoff value for the Chi-Square statistic assuming a significance
level of 0.05. In this example, with this significance level, you would not
reject the null hypothesis of independence between actual sales and before or
during because the Chi-Square statistic value was 1.90, well below the cutoff
of 7.81.
Note 1 This example comes from the long out of print text: Bell, C.E.,
Quantitative Methods for Administration, Irwin, 1977.
Results in Earlier Versions of Excel
CHIINV(p, df) is found through an iterative process that
repeatedly evaluates CHIDIST(x, df) and returns a value of x such that
CHIDIST(x, df) is "acceptably close" to p. Therefore accuracy of CHIINV depends
on two factors:
- the accuracy of CHIDIST, and
- the design of the search process and definition of
"acceptably close".
In rare cases, "acceptably close" in earlier versions
of Excel might not be close enough. This is unlikely to affect most users.
Basically, if you request CHIINV(p, df) the search continues until a value of x
is found where CHIDIST(x, df) differs from p by less than 0.0000003.
Results in Excel 2003
No changes were made in Excel 2003 to CHIDIST. The only change
affecting CHIINV was to redefine "acceptably close" in the search process to be
much closer. The search now continues until the closest possible value of x is
found (in the limits of Excel's finite precision arithmetic). The resulting x
should have a CHIDIST(x, df) value that differs from p by about 10^(-15).
Conclusions
Many inverse functions have been improved for Excel 2003. Some
have been improved for Excel 2003 only by continuing the search process to gain
a higher level of refinement. Included in this set of inverse functions are:
BETAINV, CHIINV, FINV, GAMMAINV, and TINV. No modifications were made to the
respective functions called by these inverse functions: BETADIST, CHIDIST,
FDIST, GAMMADIST, and TDIST.
Additionally, this same improvement in
the search process was made for NORMSINV in Excel 2002. For Excel 2003,
accuracy of NORMSDIST (called by NORMSINV) was improved also. These changes
also affect NORMINV and LOGINV (these call NORMSINV) and NORMDIST and
LOGNORMDIST (these call NORMSDIST).
For more information about CHIINV, click
Microsoft Excel Help on the
Help menu, type
chiinv in the
Search for box in the
Assistance pane, and then click
Start searching to view the
topic.