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Thread: Negative Trend

  1. #1
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    Negative Trend




    I am hypothesizing a decrease in a type of injury over the past decade. I have the number of admissions to my hospitals and the number of patients with the condition.

    I had originally performed a Cochran-Armitage Trend test, grouping patients based on whether they had the condition, so using proportions. I was wondering if there may be a better approach, since there are very few cases per year, say 1 out of 1500 patients or ~ 0.06%.

    I did not know if I should treat it as count data or if there may be a better way to look at it to examine a negative trend in data with a very very low prevalence of the condition.

    Thanks!
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    Re: Negative Trend

    Could you look at the time between admissions for this condition then analyze like a survival analysis? If your hypothesis were true, you should see a decreasing hazard function.

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    Re: Negative Trend

    Hmm, interesting I have not seen anything like that before. Did you just come up with that or have you seen an approach similarly used before.

    Just for a little more clarification, this is a very trauma condition that is rare, so an individual patient would not have a readmission for it. Also, I have data for 12 years, so if I played around with the PHREG approach. I do think that approach would work, but I wouldn't rule it out.

    So hypothetically say I have.

    2000: 0 / 1500
    2001: 1 / 1500
    2002: 2 / 1500
    2003: 3 / 1500
    2004: 6 / 1500
    2005: 3 / 1500
    2006: 1 / 1500
    2007: 2 / 1500
    2008: 0 / 1500
    2009: 2 / 1500

    So, I don't necessarily have incidence, just counts. Miner, you do run charts and quality engineering right? Is there some thing you all do that may work?
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    Re: Negative Trend

    There are several approaches used in quality. For binomial responses, a P-chart is used. However, there are several known issues with P-charts that occur for large sample sizes or low pBar (mean p). Your data does not have too large a sample size, but the pBar is quite low for a p-chart. When p becomes this low, a T-chart for time between rare events is more appropriate. Another consideration is that control chart limits are economically based, not statistically. Three standard deviation limits (alpha = 0.003, not 0.05) were established to balance the economic cost of chasing false alarms versus missing a process signal.

    Since you do not have the times, what about a binomial ANOM?

    I use reliability/survival analysis to analyze any events in time data. It is not limited to product failures or disease progression.
    Attached Thumbnails Attached Thumbnails Click image for larger version

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    Re: Negative Trend

    Miner did you make me graphs using my hypothetical data - thank you! I not sure if these get at trend and I just got some more years of data and even given my hypothesis, I am pretty sure a trend does not exist in my data. I will look up the ANOM figure, when I get a chance.
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    Re: Negative Trend

    Quote Originally Posted by Miner View Post
    There are several approaches used in quality. For binomial responses, a P-chart is used. However, there are several known issues with P-charts that occur for large sample sizes or low pBar (mean p). Your data does not have too large a sample size, but the pBar is quite low for a p-chart. When p becomes this low, a T-chart for time between rare events is more appropriate. Another consideration is that control chart limits are economically based, not statistically. Three standard deviation limits (alpha = 0.003, not 0.05) were established to balance the economic cost of chasing false alarms versus missing a process signal.

    Since you do not have the times, what about a binomial ANOM?

    I use reliability/survival analysis to analyze any events in time data. It is not limited to product failures or disease progression.
    wait... were those graphs done in MINITAB?!
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    Re: Negative Trend

    Quote Originally Posted by hlsmith View Post
    Miner did you make me graphs using my hypothetical data - thank you!
    Yes. This was just to illustrate using the hypothetical data.

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    Re: Negative Trend


    Quote Originally Posted by spunky View Post
    wait... were those graphs done in MINITAB?!
    Yes, they were done in Minitab.

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