Hi William, I do not have the complete explanation. But: shade var reveals missing values with time dependent distribution. Replacing "var" by "missing(var,0)" gives results independent off the order of operations. The @mod transformation should imply some division by @ngd. What @ngd is depends on the order of operations. So far, things may by not fully linear any more? Best, Martin On 03/22/2017 07:32 PM, William Kessler wrote: ```Hi Ferreters - I have a confusing difference in result depending on the order of operations. These should all be linear operations, so the order of doing them should not affect the result. I'm confused. The differences are smallish but could be significant. This is, by the way, the Ekman transport across 7.5S, and the differences are a few Sv. In one case, I first find the annual cycle, then integrate the result in x. In the other, I first integrate in x, then find the annual cycle of the result. These should be identical, but aren't (attached gif plot). I don't know which is correct. Why not? The attached cdf file (357Kb) has 5 variables: VAR original time series VAR_CL annual cycle of original VAR_INT x-integral of original VAR_INTCL annual cycle of x-integral VAR_CLINT x-integral of annual cycle These variables were defined by the following commands: yes? let/tit="original time series" var=eky[y=7.5s,x=148e:82w] yes? let/tit="annual cycle of original" var_cl=var[gt=month_irreg@mod] yes? let/tit="x-integral of original" var_int=var[x=@din] yes? let/tit="annual cycle of x-integral" var_intcl=var_int[gt=month_irreg@mod] yes? let/tit="x-integral of annual cycle" var_clint=var_cl[x=@din] yes? save/outtype=float/file=check_order_of_operations.cdf var,var_cl,var_int,var_intcl,var_clint yes? plot/line/sym=27 var_intcl,var_clint This is Ferret v6.94 (Mac) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ William S. Kessler NOAA / Pacific Marine Environmental Laboratory 7600 Sand Point Way NE Seattle WA 98115 USA william.s.kessler@xxxxxxxx Tel: +1 206-526-6221 Fax: +1 206-526-6744 ``` ``` ```