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Share Plans (Tables)
 9 Months Ended
Sep. 30, 2017
Share-based Compensation Expense
Total share-based compensation expense for the three and nine months ended September 30, 2017 and 2016 was as follows:
 
Three months ended    
 
Nine months ended
In millions
September 30,
2017
September 30,
2016
 
September 30,
2017
September 30,
2016
Restricted stock units
$
2.9

$
3.6

 
$
14.4

$
14.2

Stock options
1.9

1.8

 
8.3

9.0

Performance share units
1.4

1.0

 
9.5

5.5

Total share-based compensation expense
$
6.2

$
6.4

 
$
32.2

$
28.7
Assumptions Used in Calculating Estimated Fair Value of Stock Option Award
We estimated the fair value of each stock option award issued in the annual share-based compensation grant using a Black-Scholes option pricing model, modified for dividends and using the following assumptions:
 
2017
Annual Grant
Risk-free interest rate
1.61
%
Expected dividend yield
2.38
%
Expected share price volatility
26.9
%
Expected term (years)
6.3