Classify each of the following as a change in accounting principle or a change in accounting estimate:

a. Change from straight-line to double-declining-balance depreciation

b. Change in the uncollectibility of accounts receivable

c. Change from LIFO to FIFO for inventory

d. Change from the percentage-of-completion method to the completed-contract method for revenue on long-term construction contracts

e. Change from an 8 -year life to a 10 -year life for a machine

f. Change in estimated warranty expense stated as a percent of sales