2015 £m	2014 £m
Cost
At 1 January	207.4	205.1
Exchange differences	(0.6)	0.9
Recognised on acquisition of subsidiaries	–	1.4
Derecognised on disposal of subsidiaries	(6.2)	–
At 31 December	200.6	207.4
Accumulated impairment
At 1 January	69.0	69.4
Exchange differences	(2.2)	(0.4)
Derecognised on disposal of subsidiaries	(6.2)	–
At 31 December	60.6	69.0
Carrying amount	140.0	138.4

Goodwill acquired in a business combination is allocated, at acquisition, to the business units that are expected to benefit from that business combination. After recognition of impairment losses, the carrying amount of goodwill has been allocated to the Group's operating segments as follows:

	2015 £m	2014 £m
ADE:
Western Europe	26.4	26.5
North America	46.2	45.4
Emerging markets	–	–
AGI:
Western Europe	18.1	18.6
North America	43.7	42.1
Emerging markets	5.6	5.8
	140.0	138.4

The Group tests goodwill at least annually for impairment, or more frequently if there are indications that goodwill might be impaired.

The recoverable amounts of the cash generating units are determined from value in use calculations. The key assumptions for those calculations are the discount rates and growth rates in respect of future cash flows. Management estimates discount rates using pre-tax rates that reflect current market assessments of the time value of money and the risks specific to the cash generating units. This rate is risk adjusted, for specific countries, where the Group perceives a risk premium is appropriate. The rates used to discount the forecast cash flows for cash generating units are between 12.4% (2014: 12.3%) and 13.4% (2014: 14.3%). The recoverable amount is the sum of the discounted cash flows as forecasted for the coming five years, together with a further estimate of cash flows in perpetuity.

The forecast sales reflect management's expectation of how sales will develop at this point in the economic cycle. The expected profit margin reflects management's experience of each cash generating unit's profitability at the forecast level of sales. As outlined in the Business review, these forecasts take into account the current and expected economic environment both in respect of geography and market sectors. Cash flows after five years are based on an estimated growth rate of 3.0% (2014: 3.0%), being the historical weighted average growth in GDP in the markets that the Group operates in. Growth rates by cash generating unit range from 2.75% to 6.0%. This rate does not exceed the average long-term growth rate for the relevant markets.

If the goodwill allocated to a cash generating unit represents more than 15% of the Group's total goodwill carrying value, the cash generating unit is considered to be individually significant. The Group considers the North America ADE Heat Treatment and North America AGI Heat Treatment cash generating units to be significant cash generating units. The long term growth rates applied to cash flows after five years and the rates used to discount the forecast cash flows for these significant cash generating units are shown below:

Cash generating unit	Goodwill carrying value £m	Long term growth rate %	Discount rate %
North America ADE Heat Treatment	43.6	3.2	12.4
North America AGI Heat Treatment	43.6	3.2	12.4

The Group has conducted sensitivity analyses on the key assumptions applied to the value in use calculations for each cash generating unit. A decline in sales of 11.2% per annum in perpetuity would result in the recoverable amount of goodwill for the Group being reduced to its carrying value. The directors do not believe such a decline to be likely.

Declines in long-term growth rates of 32.1 percentage points and 6.9 percentage points for North America ADE Heat Treatment and North America AGI Heat Treatment cash generating units, respectively, would result in the recoverable amount of goodwill for these cash generating units being reduced to their carrying values. The directors do not believe such declines to be likely.

The Board has concluded that no impairment charge is required in 2015.