Related Links:
Prediction Statistics Program Page
Sample Size for Prediction Statistics Explained and Tables Page
Sample Size for Prediction Statistics Program Page
Introduction
Terminology and Formula
Examples
References
In StatsToDo, the discussions and algorithm of Prediction Statistics concerns the quality of relationship
between a binary Test and a binary Outcome.
In reality, a much wider domain of tests and outcomes exists. Outcomes that are measurements,
such as birth weight, and containing multiple categories, such as mental illness classifications, require specific and complex
multivariate methods of analysis, and are covered elsewhere. Tests that are measurements, such as maternal height and age,
requires different treatment and are considered in the Receiver Operator Characteristics (ROC) Explained Page
.
 Outcome Positive  Outcome Negative 
Test Positive  True Positive  False Positive 
Test Negative  False Negative  True Negative 
The framework under consideration for this page, in the Prediction Statistics Program Page
, and in the
Sample Size for Prediction Statistics Explained and Tables Page
, is therefore as presented in the table to the right.
 True Positive (TP) if they are test positive and outcome positive
 False Positive (FP) if they are test positive but outcome negative
 False Negative (FN) if they are test negative but outcome positive
 True Negative (TN) if they are test negative and outcome negative
The statistics of evaluating the relationship between tests and outcomes occurs under the following scenarios
 The collection of reference data, to evaluate the quality of relationship between a test and an outcome.
 Developing the parameters that can be generalized and use in clinical situation in the future
 In appropriate future situations, the use of the parameter to influence diagnostic decisions.
The terminologies and formulae for these procedures will be covered in the next panel
 Outcome Positive (O+)  Outcome Negative (O) 
Test Positive (T+)  True Positive (TP)  False Positive (FP) 
Test Negative (T)  False Negative (FN)  True Negative (TN) 
This panel presents the terminology, and formulae for calculation, for the parameters involved in Prediction
Step 1. Defining the research parameters
The Test and Outcome must be defined, as shown in the table to the right. For example, we may decide to use the observation of
an unengaged head in early labour as the Test to predict the need for a Caesarean Section as the Outcome. In this scenario, an
observed unengaged head is Test Positive (T+), and an engaged head is Test Negative (T). A baby delivered by Caesarean
Section is Outcome Positive (O+) and one delivered vaginally is Outcome Negative (O). The combination of these are as follows
 A baby with unengaged head in early labour and subsequently delivered by Caesarean Section is Test Positive (T+) and
Outcome Positive (O+), so it is a case of True Positive (TP)
 A baby with unengaged head in early labour and subsequently delivered vaginally is Test Positive (T+) and
Outcome Negative (O), so it is a case of False Positive (FP)
 A baby with engaged head in early labour and subsequently delivered by Caesarean Section is Test Negative (T) and
Outcome Positive (O+), so it is a case of False Negative (FN)
 A baby with engaged head in early labour and subsequently delivered vaginally is Test Negative (T) and
Outcome Negative (O), so it is a case of True Negative (TN)
Step 2. Collecting reference Data for evaluation
Data is collected to enable the evaluation of relationship between Test and Outcome. Although prospective collection of data
can take place, the common approach is to retrospective use of data already collected for the following reasons
 An easy access to greater volume of information
 An ability to select representative samples
 An ability to have similar number of Outcome Positives (O+) and Outcome Negative (O) so that the statistical power
for detecting both are similar
The sample size required is then determined with the help of tables in the Sample Size for Prediction Statistics Explained and Tables Page
. The
parameters required are
 The Probability of Type I Error (α), the common value of 0.05 is usually used
 The power to detect a significant predictor (1β), the common value of 0.8 is usually used
 A clinically useful prediction rate, which should be significantly better than the diagnostic equivalent of null at 0.5
Once the sample size required is calculated, this number of cases in Outcome Positive, and similar numbers of Outcome Negative,
are collected for evaluation.
Step 2. Evaluating the quality of Prediction using the data collected.
Once the data is collected, the numbers can arranged according to the table at the top right corner of the panel.
 O+ and O are the numbers of cases which are Outcome Positive and Outcome Negative
 T+ and T are the number of cases which are Test Positive and Test Negative
 TP is the number of cases that are True Positive (T+ and O+)
 FP is the number of cases that are False Positive (T+ and O)
 FN is the number of cases that are False Negative (T and O+)
 TN is the number of cases that are True Negative (T and O)
From these primary numbers two sets of parameters can be calculated. Please note that all calculation are performed by
programs in the Prediction Statistics Program Page
. The formulae listed in this panel is to assist understanding only
 Parameters of quality
 The True Positive Rate TPR = TP / O+ is the proportion of Outcome Positives that are Test Positive. TPR is also known as Sensitivity
 The True Negative Rate TNR = TN / O is the proportion of Outcome Negatives that are Test Negative. TNR is also known as Specificity
 The False Positive Rate FPR = FP / O is the proportion of Outcome Negatives that are Test Positive.
 The False Negative Rate FNR = FN / O+ is the proportion of Outcome Positives that are Test Negative.
 Mathematically, FPR = 1TNR, and FNR = 1TPR
 The statistical significance of each are calculated as follows
 For TPR, Standard Error SE = sqrt(TPR(1TPR)/O+), and one tail 95% confidence interval is >TPR1.65SE
 For TNR, Standard Error SE = sqrt(TNR(1TNR)/O), and one tail 95% confidence interval is >TNR1.65SE
 Parameters for future clinical usage
 The Likelihood Ratio for Test Positive LR+ = TPR / FPR is the ratio O+/O when Test Positive
 The Likelihood Ratio for Test Negative LR = FNR / TNR is the ratio O+/O when Test Negative
Step 3. Use of Likelihood Ratio to make clinical decisions.
The Likelihood Ratio can be used to modify the perception of risk in applicable clinical situations, using a Bayesian Probability
Algorithm
 The risk or probability of an outcome before results of the Test is known is called Pretest Probability
 The risk can be converted to Pretest Odd = Pretest Probability / (1  Pretest Probability)
 The Posttest Odd is then obtained Posttest Odd = Pretest Odd * Likelihood Ratio
 The Posttest Probability = Posttest Odd/ (1 + Posttest Odd)
If there are more than one Test to an Outcome, and providing the Tests are not tautological (so strongly correlated they are
repeat of the same test), the Posttest Probability after one Test becomes the Pretest Probability of the next Test, so that,
with increasing information, the perception of risks is modified.
This panel provides examples to demonstrate the concepts and formulations described in the two previous panels. Please Note
that the numbers in these examples are entirely artificially made up to demonstrate the procedures, and they do not reflect any real clinical information. Please also note : The numbers presented in this page are adjusted to 2 decimal places, so may be
slightly different to that produced with different rounding precision
We are midwives wishing to establish a method of assessing the risk or probability of Caesarean Section in women who are admitted
to the labour ward in early labour. Outcome Positive (O+) is Caesarean Section (CS), and Outcome Negative (O) is vaginal
delivery (VD).
Study 1 . Parity :
We wish to use the parity of the woman as the Test, as we know that women having their first baby are
More likely to require a Caesarean Section. Test Positive (T+) is nulliparous pregnancy (NP), Test Negative (T) is Multiparous
Pregnancy (MP)
Although we are uncertain initially, we feel that a diagnostic accuracy of 70% and significantly greater than 50% for both
True Positive Rate and True Negative Rate would be clinically useful. We use the commonly accepted parameter of α=0.05,
and Power = 0.8.
We use α=0.05, Power (1β)=0.8, and s=0.7, using the table in Sample Size for Prediction Statistics Explained and Tables Page
, we
will need 29 cases of Caesarean Section and 29 cases of vaginal delivery to evaluate or Test/Outcome relationship.
 CS (O+)  VD (O) 
NP (T+)  TP=12  FP=3 
MP (T)  FN=18  TN=27 
We reviewed our obstetric records of 30 women delivered by Caesarean Section, and 30 delivered vaginally, and obtained the
data as shown in the table to the right
 True Positive TP = 12, False Positive FP = 3, False Negative FN = 18, True Negative TN = 27
 True Positive Rate TPR = 12 / 30 = 0.4, SE = sqrt(0.4(0.6)/30) = 0.09 95% Confidence Interval = >0.41.65(0.09) = >0.09
 True Negative Rate TNR = 27 / 30 = 0.9, SE = sqrt(0.9(0.1)/30) = 0.06 95% Confidence Interval = >0.91.65(0.06) = >0.81
 False Positive Rate FPR = 1TNR = 0.1, False Negative Rate FNR = 1TPR = 0.6
 Likelihood Ratio Test Positive LR+ = TPR/FPR = 0.4/0.1 = 4.0
 Likelihood Ratio Test Negative LR = FNR/TNR = 0.6/0.9 = 0.67
We use the two Likelihood Ratios in a public hospital that has an overall Caesarean Section Rate of 20%
 Pretest Probability = 0.2, Pretest Odd = (0.2/(10.2)) = 0.25
 For nullipara LR+ = 4.0, PostTest Odd = 0.25*4 = 1, Posttest Probability = 1/(1+1) = 0.5
 For multipara LR = 0.67, PostTest Odd = 0.25*0.67 = 0.17, Posttest Probability = 0.17/(1+0.17) = 0.14
 In this public hospital with overall CS rate of 20%, nullipara CS rate is 50%, multipara CS rate is 14%
We use the two Likelihood Ratios in a private hospital that has an overall Caesarean Section Rate of 35%
 Pretest Probability = 0.35, Pretest Odd = (0.35/(10.35)) = 0.54
 For nullipara LR+ = 4.0, PostTest Odd = 0.54*4 = 2.15, Posttest Probability = 2.15/(1+2.15) = 0.68
 For multipara LR = 0.67, PostTest Odd = 0.54*0.67 = 0.36, Posttest Probability = 0.36/(1+0.36) = 0.27
 In this private hospital with overall CS rate of 35%, nullipara CS rate is 68%, multipara CS rate is 27%
Study 2 . Head Engagement :
We wish to use whether the head is engaged when admitted in early labour as the Test,
as we know that those with an unengaged head in early labour are more likely to require a Caesarean Section.
Test Positive (T+) is head unengaged (HU), Test Negative (T) is head engaged (HE)
Pregnancy (MP)
Although we are uncertain initially, we feel that a diagnostic accuracy of 55% and significantly greater than 50% for both
True Positive Rate and True Negative Rate would be clinically useful. We use the commonly accepted parameter of α=0.05,
and Power = 0.8.
We use α=0.05, Power (1β)=0.8, and s=0.55, using the table in Sample Size for Prediction Statistics Explained and Tables Page
, we
will need 122 cases of Caesarean Section and 122 cases of vaginal delivery to evaluate or Test/Outcome relationship.
 CS (O+)  VD (O) 
HU (T+)  TP=39  FP=26 
HE (T)  FN=91  TN=104 
We reviewed our obstetric records of 130 women delivered by Caesarean Section, and 130 delivered vaginally, and obtained the
data as shown in the table to the right
 True Positive TP = 39, False Positive FP = 26, False Negative FN = 91, True Negative TN = 104
 True Positive Rate TPR = 39 / 130 = 0.3, SE = sqrt(0.3(0.7)/130) = 0.04 95% Confidence Interval = >0.31.65(0.04) = >0.23
 True Negative Rate TNR = 104 / 130 = 0.8, SE = sqrt(0.8(0.2)/130) = 0.04 95% Confidence Interval = >0.81.65(0.04) = >0.74
 False Positive Rate FPR = 1TNR = 0.2, False Negative Rate FNR = 1TPR = 0.7
 Likelihood Ratio Test Positive LR+ = TPR/FPR = 0.3/0.2 = 1.5
 Likelihood Ratio Test Negative LR = FNR/TNR = 0.7/0.8 = 0.88
We use the two Likelihood Ratios in a public hospital that has an overall Caesarean Section Rate of 20%
 Pretest Probability = 0.2, Pretest Odd = (0.2/(10.2)) = 0.25
 For unengaged head LR+ = 1.5, PostTest Odd = 0.25*1.5 = 0.38, Posttest Probability = 0.38/(1+0.38) = 0.27
 For engaged head LR = 0.88, PostTest Odd = 0.25*0.88 = 0.22, Posttest Probability = 0.22/(1+0.22) = 0.18
 In this public hospital with overall CS rate of 20%, those with unengaged head in early labour have CS rate of 27%,
those with engaged head 18%
We use the two Likelihood Ratios in a private hospital that has an overall Caesarean Section Rate of 35%
 Pretest Probability = 0.35, Pretest Odd = (0.35/(10.35)) = 0.54
 For unengaged head LR+ = 1.5, PostTest Odd = 0.54*1.5 = 0.81, Posttest Probability = 0.81/(1+0.81) = 0.45
 For engaged head LR = 0.88, PostTest Odd = 0.54*0.88 = 0.47, Posttest Probability = 0.47/(1+0.47) = 0.32
 In this private hospital with overall CS rate of 35%, those with unengaged head in early labour have CS rate of 45%,
those with engaged head 32%
Study 3 . Combining the two : From the previous two studies we know the following
 For parity, LR+ (nullipara) = 4.0, LR (multipara) = 0.67
 For head engagement, LR+ (head unengaged) = 1.5, LR (head engaged) = 0.88
In the public hospital with an overall Caesarean Section Rate of 20%
 Pretest Probability = 0.2, Pretest Odd = (0.2/(10.2)) = 0.25
 For nullipara PostTest Odd = 1, Posttest Probability = 0.5. This can be used as the Pretest Probability and Odd for the next stage
 For unengaged head LR+ = 1.5, PostTest Odd = 1*1.5 = 1.5, Posttest Probability = 1.5/(1+1.5) = 0.60
 For engaged head LR = 0.88, PostTest Odd = 1*0.88 = 0.88, Posttest Probability = 0.88/(1+0.88) = 0.47
 For multipara PostTest Odd = 0.17, Posttest Probability = 0.14
 For unengaged head LR+ = 1.5, PostTest Odd = 0.17*1.5 = 0.24, Posttest Probability = 0.24/(1+0.24) = 0.20
 For engaged head LR = 0.88, PostTest Odd = 0.17*0.88 = 0.14, Posttest Probability = 0.14/(1+0.14) = 0.13
In the private hospital with an overall Caesarean Section Rate of 35%
 Pretest Probability = 0.35, Pretest Odd = (0.35/(10.35)) = 0.54
 For nullipara PostTest Odd = 2.15, Posttest Probability = 0.68. This can be used as the Pretest Probability and Odd for the next stage
 For unengaged head LR+ = 1.5, PostTest Odd = 2.15*1.5 = 3.23, Posttest Probability = 3.23/(1+3.23) = 0.76
 For engaged head LR = 0.88, PostTest Odd = 2.15*0.88 = 1.90, Posttest Probability = 1.90/(1+1.90) = 0.68
 For multipara PostTest Odd = 0.36, Posttest Probability = 0.27
 For unengaged head LR+ = 1.5, PostTest Odd = 0.36*1.5 = 0.54, Posttest Probability = 0.54/(1+0.54) = 0.35
 For engaged head LR = 0.88, PostTest Odd = 0.36*0.88 = 0.32, Posttest Probability = 0.32/(1+0.32) = 0.24
  Pretest Probability  Likelihood Ratio  Posttest Probability 
Public Hospital  Nullipara  0.20  4.00  0.50 
 Multipara  0.20  0.67  0.14 
 Unengaged Head  0.20  1.50  0.27 
 Engaged Head  0.20  0.88  0.18 
 Nullipara+Unengaged Head  0.50  1.50  0.60 
 Nullipara+Engaged Head  0.50  0.88  0.47 
 Multipara+Unengaged Head  0.14  1.50  0.20 
 Multipara+Engaged Head  0.14  0.88  0.13 
Private Hospital  Nullipara  0.35  4.00  0.68 
 Multipara  0.35  0.67  0.27 
 Unengaged Head  0.35  1.50  0.45 
 Engaged Head  0.35  0.88  0.32 
 Nullipara+Unengaged Head  0.68  1.50  0.76 
 Nullipara+Engaged Head  0.68  0.88  0.65 
 Multipara+Unengaged Head  0.27  1.50  0.35 
 Multipara+Engaged Head  0.27  0.88  0.24 
Summary
The table to the right shows how the Likelihood ratios can be used to modify diagnosis, in terms of probability,
and how multiple tests can be sequentially integrated, so that clinical decisions can be made and modified as additional
information becomes available.
The end results is independent of how the sequence is arranged, so that using the unengaged head to modify decisions made
with parity, or using parity to modify decisions made with unengaged head, will produce the same results. The only thing users
need to be careful about is that, when multiple Tests are used, each should represent an independent predictor. Tests so
closely correlated that they represents multiple versions of the same thing leads to inappropriate weighting of some predictors,
and will produce misleading results.
Sensitivity and specificity :
Practical Statistics for medical Research. (1994) F.Altman. Chapman Hall, London.
ISBN 0 412 276205 (First Ed. 1991) p.409417
Likelihood Ratio :
Simel D.L., Samsa G.P., Matchar D.B. (1991) Likelihood ratios with confidence:
sample size estimation for diagnostic test studies. J. Clin. Epidemiology
vol 44 No. 8 pp 763770
Pre and post test probability :
Deeks J.J, and Morris J.M. (1996) Evaluating diagnostic tests. In Bailliere's
Clinical Obstetrics and Gynaecology Vol.10 No. 4, December 1996
ISBN 0702022608 p. 613631.
Fagan T.J. (1975) Nomogram for Bayer's Theorem. New England J. Med. 293:257
General :
Sackett D, Haynes R, Guyatt G, Tugwell P. (1991) Clinical Epidemiology: A Basic
Science for Clinical Medicine. Second edition. ISBN 0316765996.
Sample size :
Beam, C. A. (1992), "Strategies for Improving Power in Diagnostic Radiology
Research," American Journal of Radiology, 159, 631637.
Casagrande, J. T., Pike, M. C., and Smith, P. G. (1978), "An Improved
Approximate Formula for Calculating Sample Sizes for Comparing Two Binomial
Distributions," Biometrics, 34, 483486.
