Sampling methodology and sample size

Samples were collected from a specialized cardiovascular hospital the most extensive and only reference for patients with CVDs using a purposive random sampling method. Initially, a list of all patients admitted to the study during the two intervals was prepared, and data were collected from inpatients medical records through a checklist. If necessary, patients who were eligible to participate in the study were contacted, and additional information was obtained via telephone interview.

Four types of CVDs with high frequency (coronary artery stenosis, heart failure, heart attack, and cardiac rheumatism) were selected to estimate sample size. Then, the medical records of 40 discharged patients were randomly selected and evaluated for cost. Based on the mean and variance of hospital costs and using Cohen’s sampling method, the sample size was estimated to be 253 (n = 800) before and 284 (n = 1300) after the HTP.

Cohen’s d is simply a measure of the distance between two means, measured in standard deviations [23]. It is calculated using Eq. (1):

where M₁ and M₂ are the means for the 1st and 2nd samples, and SD_pooled is the pooled standard deviation for the samples. SD_pooled is appropriately calculated using the following equation:

Accordingly, the total number of hospitalizations for CVDs was extracted from the hospital information system (HIS), then, 600 patients for both intervals (300 before and 300 after the HTP) were randomly selected using random number table.

A pilot study was conducted a month before (August 2019) on 30 inpatients with CVDs by a face-to-face interview to estimate indirect costs in the last month. The data were extracted using a checklist. Data were multiplied by 12 to estimate the total costs in the last year. Based on the mean and variance of hospital costs and using Cohen’s sampling method, the sample size was estimated to be 180 for both time points, before and after the HTP implementation. By telephone follow-up, patients were selected from the direct costs calculation phase (n = 300). Contact with all selected patients continued until the designated sample size was obtained.