Description NNT Interpretation Perfect treatment, previously untreatable condition with no placebo effect involved 0.0 1.0 1 Half of participants receive the treatment, and half receive a control (which may be simply a placebo, or may be an existing treatment with a known effectiveness). Every person that receives the treatment shows improvement, which may be a reduction or halt in worsening of the condition, an improvement in the condition, or an outright cure of the condition. Every person in the control group shows no improvement, therefore the condition never improves on its own and the control is never effective. NNT is 1/(1.0-0.0), which is 1. Very effective treatment with large improvement over control 0.1 0.9 1.25 For simplicity, a low number of participants will be used, thought scientific studies almost always require many more. Ten people receive the treatment, and ten receive a control. Of the ten in the treated group, nine show improvement, and one shows no improvement. In the control group, one person shows improvement and nine show none. Since one of those who received the control showed improvement without the treatment, it is said that one of the nine from the treated group would have improved without receiving the treatment. Therefore, one person’s outcome does not represent evidence that the treatment is better than the control. NNT is 1/(0.9-0.1), which is 1.25. The absolute risk reduction is 0.9-0.1, equal to 0.8. Effective treatment with moderate improvement over control 0.3 0.7 2.5 Ten receive the treatment, and ten receive a control. In the treatment group, seven show improvement and three show none. In the control group, three show improvement and seven show none. Therefore, the treatment was more helpful than the control in three of ten cases  , and was not any more helpful in seven . NNT is 1/(0.7 – 0.3), which is 2.5. Effective treatment, but little improvement over control 0.4 0.5 10 Ten receive the treatment, and ten receive a control. Six improve with the treatment, and four do not. In the control group, five improve and five do not. Therefore, the treatment was more helpful than the control in only one of ten cases  , and was not helpful is nine of ten  . NNT is 1/(0.5 – 0.4), which is 10. Not very effective treatment with little improvement over control 0.8 0.9 10 Ten receive the treatment, and ten receive a control. Two improve with the treatment and eight do not. In the control group, one improves and nine do not. Therefore, the treatment was more helpful than the control in only one of ten cases, and was not helpful is nine of ten. NNT is 1/(0.9 – 0.8), which is 10. Very effective treatment, but with little  improvement over control 0.1 0.2 10 Ten receive the treatment, and ten receive a control. Nine improve with the treatment and one does not. In the control group, eight improve and two do not. Therefore, the treatment was more helpful than the control in only one of ten cases, and was not helpful is nine of ten. NNT is 1/(0.2 – 0.1), which is 10. Treatment is  effective but worse than control 0.2 0.1 −10 Ten receive the treatment, and ten receive a control. Eight improve with the treatment and two do not. In the control group, nine improve and one does not. Therefore, the treatment was less helpful than the control in one of ten cases. NNT is 1/(0.1 – 0.2), which is -10. Notice that, even though the treatment was effective in eight of ten cases (only one less than the previous example) the NNT has shifted from 10 to -10. This is because NNT measures how many patients must be given the treatment instead of the control in order to see improvement in one person. Since giving the treatment to ten people would cause one of those people to be worse than if they had received the control instead, the NNT is -10. 
Description NNT Interpretation Perfect treatment, previously untreatable condition with no placebo effect involved 0.0 1.0 1 Half of participants receive the treatment, and half receive a control (which may be simply a placebo, or may be an existing treatment with a known effectiveness). Every person that receives the treatment shows improvement, which may be a reduction or halt in worsening of the condition, an improvement in the condition, or an outright cure of the condition. Every person in the control group shows no improvement, therefore the condition never improves on its own and the control is never effective. NNT is 1/(1.0-0.0), which is 1. Very effective treatment with large improvement over control 0.1 0.9 1.25 For simplicity, a low number of participants will be used, thought scientific studies almost always require many more. Ten people receive the treatment, and ten receive a control. Of the ten in the treated group, nine show improvement, and one shows no improvement. In the control group, one person shows improvement and nine show none. Since one of those who received the control showed improvement without the treatment, it is said that one of the nine from the treated group would have improved without receiving the treatment. Therefore, one person’s outcome does not represent evidence that the treatment is better than the control. NNT is 1/(0.9-0.1), which is 1.25. The absolute risk reduction is 0.9-0.1, equal to 0.8. Effective treatment with moderate improvement over control 0.3 0.7 2.5 Ten receive the treatment, and ten receive a control. In the treatment group, seven show improvement and three show none. In the control group, three show improvement and seven show none. Therefore, the treatment was more helpful than the control in four of ten cases (7 treated improved minus 3 controls improved) , and was not any more helpful in six of ten cases (3 not improved despite treatment, 3 that would have improved anyway as seen in the control group) . NNT is 1/(0.7 – 0.3), which is 2.5. Effective treatment, but little improvement over control 0.4 0.5 10 Ten receive the treatment, and ten receive a control. In the treatment group six improve with the treatment, and four do not. In the control group, five improve and five do not. Therefore, the treatment was more helpful than the control in only one of ten cases (6 treated improved minus 5 controls improved) , and was not helpful is nine of ten (4 not improved despite treatment, 5 that would have improved anyway as seen in the control group) . NNT is 1/(0.5 – 0.4), which is 10. Not very effective treatment with little improvement over control 0.8 0.9 10 Ten receive the treatment, and ten receive a control. Two improve with the treatment and eight do not. In the control group, one improves and nine do not. Therefore, the treatment was more helpful than the control in only one of ten cases, and was not helpful is nine of ten. NNT is 1/(0.9 – 0.8), which is 10. Apparently very effective treatment, but with little real improvement over control 0.1 0.2 10 Ten receive the treatment, and ten receive a control. Nine improve with the treatment and one does not. In the control group, eight improve and two do not. Therefore, the treatment was more helpful than the control in only one of ten cases, and was not helpful is nine of ten. NNT is 1/(0.2 – 0.1), which is 10. Treatment is very effective but worse than control 0.2 0.1 −10 Ten receive the treatment, and ten receive a control. Eight improve with the treatment and two do not. In the control group, nine improve and one does not. Therefore, the treatment was less helpful than the control in one of ten cases. NNT is 1/(0.1 – 0.2), which is -10. Notice that, even though the treatment was effective in eight of ten cases (only one less than the previous example) the NNT has shifted from 10 to -10. This is because NNT measures how many patients must be given the treatment instead of the control in order to see improvement in one person. Since giving the treatment to ten people would cause one of those people to be worse than if they had received the control instead, the NNT is -10. If control is placebo, giving placebo appear better than to give treatment.
R
three
four
A
(7 treated improved minus 3 controls improved)
R
seven
six of ten cases (3 not improved despite treatment, 3 that would have improved anyway as seen in the control group)
R
Six
In the treatment group six
A
(6 treated improved minus 5 controls improved)
A
(4 not improved despite treatment, 5 that would have improved anyway as seen in the control group)
R
Very
Apparently very
A
real
A
very
A
If control is placebo, giving placebo appear better than to give treatment.