Calculating trend rate of change

Hi everyone,

I'm currently analysing a time series of data (see code below). I've fitted a trendline into it but I would like to calculate its rate of change per year. Any ideas how can it be done?

Thanks in advance,

Maja

%script to plot year-by-year data points for the full basin, fit a
%trendline and test it for significance with Mann-Whitney U test 

%load data
data=readtable('Mydata.csv');

%define variables
year=data{:,1};
wind=data{:,8};

figure (1)

plot(year,wind,'b','LineWidth',1.2)
xlabel('\bf Year')
ylabel('\bf Wind speed (m/s)')
grid on
hold on

%fit a trendline to the data
wind_coeff=polyfit(year,wind,1);
wind_trend=polyval(wind_coeff,year);
plot(year,wind_trend,'r--','LineWidth',1.2);

%this bit tests the trend for significance with Mann-Whitney
%if p is very small and h=1, trend is significant
[p,h]=ranksum(year,netwind);

Answer 1

[Bart Kieviet]

I guess you mean the derivative? It's the function

diff

More on this function here.

[Maja Zdulska]

Sorry, I just realized I didn't make myself clear.

I'm not looking for the change between each pair of neighbouring elements, but rather for the rate of change per year averaged between all elements (so the answer would only be one number).

[Bart Kieviet]

If I understand correctly you have a year-by-year numbers, so say:

year = [2010, 2011, 2012, 2013];
wind = [10, 15, 5, 16];

if I run diff, I get:

df = diff(wind); % [5, -10, 11]

I could use mean to get the average:

mean( df ); % 2

[Maja Zdulska]

Ah, I wasn't sure whether I could just do it like that. Thanks a lot!