Plotly: what's the difference between ols and lowess trendlines

Question

Plotly: what's the difference between ols and lowess trendlines

dlanced

2021年9月22日 11:19

I'm plotting a dataset with a trendline, but using ols vs lowess gives me significantly different line shapes. I'm sure this is an expected and perfectly normal result, but I'm afraid I don't understand the significance of the difference. Could someone explain what's going on? Here's my code using ols:

fig = px.scatter(df_small, y=Ratio,
                 trendline=ols, 
                 trendline_color_override=red)
fig.show()

And here's the plot it produced:

Using lowess gave me this plot: For context, my dataset represents the ratio of daily Covid deaths:new infections in the Province of Ontario over the previous 18 months.

Topic plotly

Category Data Science

Erwan · Accepted Answer · 2021年9月22日 11:19

I was curious so I looked it up:

The first method OLS is actually Ordinary Least Squares regression so it just fits a linear regression line. It's probably not what you want in this case since it cannot fit the waves (unless you just want to show the overall trend).
The lowess method is local polynomial regression. Since plotly 5.2.1 trendline_option is available to lowess trendlines which can make the trendline follow the underlying data more closely.

For the record I did a small test with R and ggplot2 which seems to have a better implementation:

library(ggplot2)
library(plyr)

t<-seq(0,12,.1)
noise<-rnorm(121,0,.25)
v<-cos(t)+noise
d<-data.frame(x=t,y=v)
ggplot(d,aes(x,y))+geom_point()+ geom_smooth(method="loess")

Plotly: what's the difference between ols and lowess trendlines

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