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Share Plans (Tables)
 3 Months Ended
Mar. 31, 2017
Share-based Compensation Expense
Total share-based compensation expense for the three months ended March 31, 2017 and 2016 was as follows:
 
Three months ended
In millions
March 31,
2017
March 31,
2016
Restricted stock units
$
5.8

$
7.0

Stock options
4.5

5.4

Performance share units
6.1

3.7

Total share-based compensation expense
$
16.4

$
16.1
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