once a year
You should change them once a year. We have the same lights, just to be sure it doesn't hurt to change them every year.
The base-year value is ordinarily shown as a percentage with the percentage symbol omitted, often 100.0. An example might be Base Year percentage 100.0 and Current Value 139.9.
It depends on the year.
once a year
twice a year
You normally divide by the previous year. The general rule for calculating a relative (or percentage) change is to look at the change compared to the base, or earlier, year.
Many times a change in the base sequence during DNA replication will cause a mutation but if it is the 3rd base often there will be no mutation . It will often give the same protein. The term is genetic "wobble" and is seen when mRNA and tRNA match do not match at the third base.
Variable annuity insurance is insurance that has a variable year to year and it can change upon facts that change such as your base description of how you manage your life.
You protect derived classes from breaking when you change the internal parts of the base class by making sure that the public (or protected) parts of the base class do not change. Only the private parts may change. If the public interface changes, then the derived class must often change.
result of a series of numbers related to the base number as in a percent change over years related to the base year
The prime rate normally changes only when the national banks decide to raise or lower their base rate. The prime rate has been known to not change for years but it has also been known to change several times in one year. http://en.wikipedia.org/wiki/Wall_Street_Journal_prime_rate