chapter  12
17 Pages

Fluctuations and trends

Runningmeans Withdifficultcasessuchasthis,thereareotherdevicesthatmaybeused tosimplifythetaskofjudgementandassessment.Thefirstoftheseaimsat smoothingoutthesharpandmarkedirregularitiesthatcanbeseenin Figs.42and43sothatonlythemajorfluctuationsarestressedandsoneed

beconsidered.Thiscanbeeffectedbythecalculationof'runningmeans'. Thisimpliesthatif'five-yearrunningmeans'arebeingused,forexample, thenthefirstvaluewillbetheaverageofyears1-5;thesecondvaluewill betheaverageofyears2-6;thethirdvaluewillbetheaverageofyears3-7, etc.,untilthefinalfiveyearsoftheperiod.FortheBidstondatathefirst twovalueswouldbeasfollows:

1901640 1902649 1903874682·6 1904640697·2 1905610 1906713

Anynumberofyearsmaybethebasisforsuchasmoothingtechnique, butitmustbeborneinmindthatifthereweretobearegularperiodicity inthefluctuationsofthesamelengthastherunning-meanperiod,then sucharegularfluctuationwouldnotappearintheresultantgraphs.Itis thereforeusuallydesirabletopreparesuchgraphsfortwoormoreperiods ofdifferentlengths.TheseBidstondataareherepresentedonlyforthe 'five-year'periodofrunningmeans(Fig.44)andvaluesseemmarkedlyto increaseaftertheperiod1913-1917(mid-year1915),althoughsmaller fluctuationsareseentooccurbothbeforeandafterthisperiod.