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Unit 9 introduction to ih statPresentation Transcript
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Introduction to IHStat software(requiredfor assignment 3)
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IHStat is a free statistical package developed by the American Industrial Hygiene Association for evaluating occupational samples The program is a preprogramed Excel worksheet that: computes descriptive statistics determines if the distributions are normal or log normally distributed calculates associated confidence limits Ideal for determining compliance with little work What is ihstat?
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Normal distribution When data resembles a bell shape (i.e. equally clustered around the mean/average) Often occurs with noise data and/or physiological monitoring (ex. blood pressure, heart rate) Log Normal distribution When original data is skewed and does not resemble a bell curve Occurs frequently with air monitored sampling (e.g. particulates) Transforming the data (i.e. taking the natural logarithm of each sample) provides us with a normally distributed variable Lognormal vs normal distributions
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The remainder of this PowerPoint will walk you through how to use IHStatwith the use of screen shots. Before we can proceed you need to: Download and open IHStat from Moodle or from the AIHA website Ensure macros are enabled in Excel NOTE: if you are unable to enable macros there is a macro free version available on the AIHA website; however, the layout and design is slightly different. How do I use Ihstat?
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Opening page of ihstat program STEP 2: Click button to enter program STEP 1: Select language preference
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How to enter exposure limit for comparisons STEP 3: Enter /change occupational exposure limit of hazard you are evaluating by typing in the new value. For example: chlorine has a TLV of 0.5 ppm.
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How to clear data STEP 4: When you open IHSTAT there may be existing data in the sheet. Clear existing data to input new data by clicking on the eraser (circled).
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How to Enter data STEP 5: Enter your data Copy the data points from your Excel sheet you want to evaluate
Data must be copied from one column
Go to IHStat program, click the first empty cell under the “Sample data” column and paste data (or enter by hand) When you paste your data the statistics/ graphics will be computed
NOTE: Max 200 data points can be entered into program
This first box provides the descriptive statistics you will want to include into your reports such as the number of samples, mean, standard deviation, and geometric mean and standard deviation (i.e. mean of log transformed data) , etc.
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You will also note they include the percent of samples above the OEL you indicated in Step 3
evaluating datadistributions Step 7: Determine the distribution of your data The second data box helps you determine if your data fits a normal (i.e. bell curve) or a lognormal distribution (i.e. skewed curve). This is important as the confidence limits used to determine compliance are computed differently. IHStat makes determining distributions easy. They provide you with the W-test statistic, but also provide you with a simple /easy way to interpret the results:
Yes/blue thumbs up = fits distribution
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No/red thumbs down = does not fit distribution
In this example we can see that the data fits a log normal distribution.
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α level NOTE: α = 0.05 is referring to the confidence level α = 0.05 means we have a confidence level of 95% IHStat uses 95% confidence level for all tests
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Choosing confidence limits What do you do in cases where: Data fits both a normal and lognormal distribution
In this situation you can choose either option however, a normal distribution is easier to interpret
Data does not fit a normal or lognormal distribution
In this type of situation you are best using a nonparametric analysis method, which cannot be done in IHStat
obtaining confidence limits Step 8: Obtaining confidence limits (i.e. UCL, LCL) Now that you know which distribution your data follows you can select the confidence limits provided in either the: Lognormal parametric statistics box
Use this box if your data fits a lognormal distribution from Step 7
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Confidence limits are calculated by Land’s Exact method
Normal parametric statistics box
Use this box if your data fits a normal distribution from Step 7
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Confidence limits are calculated by t-statistics
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saving files You can save the program with the current data by clicking the save icon on the program sheet.
NOTE: this will save the file to a default location and does not allow you to change the name of the file so you continually overwrite the existing data.
When you are doing your assignment it is a good idea to save each analysis (i.e. each grouping) as a separate file so you don’t have to reenter data if you want to double check your numbers.
To save each analysis as it’s own file go into the file menu and select “save as”
Imbedded notes/comments Please note that in some of the data result boxes there are small red triangles.
These indicate that there are comments associated with the test
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To view comments hover over triangle and the comment will appear (as shown above)
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These comments are meant to provide you with information or descriptions of the statistic