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tokens: is particularly the case when considering stars with (quasi)periodic amplitude/phase change, commonly known as the Blazhko phenomenon ( Blazhko 1907 , see also Shapley 1916 ). Most of the progress made on this issue comes from observations, largely attributed to the space missions CoRoT and Kepler (e.g., Benkő et al. 2014 ; Szabó et al. 2014 ). Nevertheless, even these highly accurate data have been unable to provide the grounds for a physically sound (and working) scenario 1 . There are also contradictory figures on the mere occurrence rates of the Blazhko (hereafter BL) stars. These rates (for fundamental mode stars) range from ~ 12% to ~ 50% (see Kovacs 2016 and Smolec 2016 and references therein). Interestingly, analyses of the high-precision space observations yield apparently quite similar rates to those derived from classical ground-based data ( Benkő et al. 2014 ; Jurcsik et al. 2009 ). This, and the apparent lack of application of modern methods of time series analysis in this field, has prompted us to examine the occurrence rates of the BL stars from a subset of the available space data. Even without knowing the underlying physical cause of the BL phenomenon, it is clear that this single number may help to distinguish between various ideas. For example, models based on the resonance between radial modes will likely face difficulties if amplitude/phase modulation is indeed as common as the present study shows. 2. Datasets and the method of analysis In establishing a solid, but not too extensive dataset for this introductory work, we opted for the Kepler two-wheel (K2) survey. Unfortunately, there are not many well-documented (star-by-star) RR Lyrae surveys available for this mission. However, the variability survey on Campaigns 0–4 by Armstrong et al. (2016) has proven to be extensive enough, suitable for the purpose of this work. We have chosen not to include C0 due to the short observational time span devoted to this campaign in the early phase of K2. A few basic parameters of the datasets used are listed in Table 1 . As this table shows, there are several miss-classifications in the original list 2 . Nevertheless, the number of the remaining stars is large enough to use them as a statistically significant sample in assessing the occurrence rate of the BL phenomenon. There are some four or five stars belonging to the globular cluster M4 in C02; otherwise the rest are apparently Galactic field/halo stars (many of them are rather faraway objects); see Fig. 1 for the distribution of the 237 targets analyzed in this paper over the Kepler field of view during the four campaigns. Fig. 1. Distribution of the stars analyzed in this letter (see Table 1 ) in the field of view of Kepler . Table 1. Summary of the datasets used in this paper. Notes. N tot stands for the total number of stars classified as RRab stars by Armstrong et al. 2016 . N RR is for the actual number of RRab stars. The time series data were downloaded for C01 and C02 from