The phrase "approximately a linear relationship" is correct and usable in written English.
It can be used in contexts where you are discussing the nature of a relationship between two variables that is close to linear but not perfectly so.
Example: "The data suggests that there is approximately a linear relationship between temperature and ice cream sales, indicating that as temperatures rise, sales tend to increase."
Alternatives: "roughly a linear relationship" or "nearly a linear relationship".
The residual strength of the fatigue-cracked tubular joints follows approximately a linear relationship with respect to the percentage of the crack area along the brace-to-chord intersection.
Adelgid populations responded to imidacloprid dosage with approximately a linear relationship between the percent population reduction (probability scale) vs. log of dosage; 50% reduction in populations could be achieved with 0.15 g per 2.5 cm of trunk diameter at breast height (dbh), or 10% of the maximum labeled dosage.
In addition, there was approximately a linear relationship between a f and log SNR).
As shown, the pressure gradient increases with the rise of flow rate, which approximately keeps a linear relationship.
Finally, it approximately seems a linear relationship as same as Darcy's law (Huang et al. 2016; Huang et al. 2013; Xiong et al. 2017).
The expression levels of five miRNAs corresponded approximately to a linear relationship between the glycaemic phenotypes of the three strains.
For exon microarray, the mRNA expression levels of all three genes followed an approximately linear relationship with sample purity, whereas larger fluctuations around a linear relationship were observed for all three genes in tag-seq.
Similarly, the plot displays a linear relationship with a slope of approximately −1 (thus −1.049), indicating that cation exchange is a significant process in the study area.
In a linear relationship, it would have been approximately the same difference in logits between each adjacent pair of course grades.
A linear relationship of the Goodman type can approximately be fitted to the constant fatigue life data for GLARE-2, regardless of the fiber orientations.
Although we observed a linear relationship between sorafenib dose in vivo and tumor growth delay, 100 mg/kg was toxic to approximately 10% of animals.
