ReproNim Statistics Module

The positive Predictive Value


Teaching: 10 min
Exercises: 20 min
  • What is the positive Predictive Value (PPV) ?

  • After this lesson, you should understand what is the PPV

Definitions: remiders

PPV : definition

PPV = P(H_A \mid T_S)

PPV : How do I compute it ? where does it come from ?

Let’s do some basic probability

We consider that the hypotheses H_0 and H_A are random, i.e. they have associated probabilities. For instance, the probability of H_0 to be true could be 20%. We have either H_A is true, or else P_0 is true. Therefore:

P(H_A = True) + P(H_0 = True) = 1.

We simply note

P(H_A = True) as P(H_A)


P(T = T_S) as P(T_S)

We are interested in the probability of a significant test, we can write

P(T_S) = P(T_S, H_A) + P(T_S, H_0)

because $T_S$ occurs either under the null or the alternative (mutually exclusive) and their represent all the possibilities.

Bayes theorem

The famous theorem Bayes theorem states:

P(A, B) = P(A \mid B) P(B)

and therefore

P(A \mid B) = \frac{P(A, B)}{P(B)} = \frac{P(B \mid A) P(A)}{P(B)}

Bayes theorem in image

Apply this to our question:

Now, apply this to the probability of the test results T.

The probability of a significant result of the test T=T_S is :

P(T_S) = P(T_S, H_A) + P(T_S, H_0)
P(T_S) = P(T_S \mid H_A) P(H_A) + P(T_S \mid H_0) P(H_0)

What is the probability of H_A given that the test is significant (eg, PPV) ?

P(H_A \mid T_S) = \frac{P(T_S \mid H_A) P(H_A)}{P(T_S)} = \frac{P(T_S \mid H_A) P(H_A)}{P(T_S \mid H_A) Pr(H_A) + Pr(T_S \mid H_0) Pr(H_0)}

And we know that

P(T_S \mid H_A) = 1 - P(T_N \mid H_A) = 1 - \beta = W


P(H_A \mid T_S) = \frac{W P(H_A)}{W P(H_A) + \alpha P(H_0)}

Defining: R = \frac{P(H_A)}{P(H_0)} the odd ratio of the alternative over the null

P(H_A \mid T_S) = \frac{W R}{W R + \alpha}

PPV :Some exercises

Task: Play with the PPV - understand the impact of the parameters

Pick a recent study that you have done in fMRI or using anatomical data.  
try to propose values for power, alpha, and prior
Vary power from .1 to .9 and print or plot results

First define a function to compute the PPV from power, odd ratio and alpha
The solution below is in Python, but feel free to do it in your favorite scripting language


def PPV_OR(odd_ratio, power, alpha, verbose=True):
    returns PPV from odd_ratio, power and alpha
    odd_ratio: float
    power: float
        Power for this study
    alpha: float
        type I risk of error
        The positive predicted value
    ppv = (power*odd_ratio)/(power*odd_ratio + alpha)
    if verbose:
        print("With odd ratio=%3.2f, "
               "Power=%3.2f, alpha=%3.2f, "
               "We have PPV=%3.2f" %(odd_ratio,power,alpha,ppv))
    return ppv
Second define a function to display easily the results 


def plot_ppv(xvalues, yvalues, xlabel, ylabel, title):
   simply plot yvalues against xvalues, with labels and title
   xvalues, yvalues : iterables of numbers 
   labels and title : string
   fig = plt.figure();
   axis = fig.add_subplot(1, 1, 1)
   axis.plot(xvalues, yvalues, color='red', marker='o', linestyle='dashed',
           linewidth=2, markersize=14);
   axis.set_title(figure_title, fontsize=20);
   return fig, axis
Last play with parameters : first, let's vary power


# An example
R = 1./5.
Pw = .5
alph = .05
ppv = PPV_OR(R, Pw, alph)

# Vary power:
Pw = np.arange(.1,1,.2)
ppvs = [PPV_OR(R, pw, alph, verbose = False) for pw in Pw]
xlabel = 'Power'
ylabel = 'PPV'
figure_title = 'With an odd ratio of {odd_ratio}'.format(odd_ratio=R)

# print
plot_ppv(Pw, ppvs, xlabel, ylabel, figure_title);
Then, let's vary odd ratio 


# Vary odd ratio:
Pw = .5
alph = .05
odd_ratios = np.arange(.05,.5,.05)
ppvs = [PPV_OR(R, Pw, alph, verbose = False) for R in odd_ratios]

# print
figure_title = 'With a power of {power}'.format(power=Pw)
xlabel = 'odd_ratios'
ylabel = 'PPV'
plot_ppv(odd_ratios, ppvs, xlabel, ylabel, figure_title);

Last, let's vary alpha, remember that p-hacking may give us large type I risk of errors 


# Vary alpha:
Pw = .5
R = 1/5
alphas = np.arange(0, .2, 0.01)# [0.001, .005, 0.01, 0.05, 0.1] #, 0.2, 0.3, 0.4, 0.5]
ppvs = [PPV_OR(R, Pw, alph, verbose = False) for alph in alphas]

# print
xlabel = 'alpha'
ylabel = 'PPV'
figure_title = 'With a power of {power} and odd ratio of {odd_ratio}'.format(
                                        power=Pw, odd_ratio=R)
plot_ppv(alphas, ppvs, xlabel, ylabel, figure_title);

This is also in a python notebook

The notebook is : PPV

How to work with the notebooks ?

There are two cases.

  1. You do not really know python, and how to install the jupyter notebook. You can still read the notebook, skipping the code sections. The notebook will introduce some definitions, and then play with different settings. But the true benefit comes if you can install the jupyter project see here and actually play with the code, for instance changing the sample size or effect size to understand better what power is.

  2. Download the notebook, and try to understand the concepts and the code. If the code is unclear, please make an issue on the repronim github site

Key Points