Mma Ramotswe thought for a moment. She was no statistician, but she had read Clovis Andersen on the subject of unusual occurrences. The fact that something happens once, the author of The Principles of Private Detection had written, does not mean that it will happen again. And remember that some events are pure one-offs. They are freaks. They are coincidences. Don’t base a whole theory on them. Clovis Andersen was probably right in general, and if he was also right in this particular case, then there was nothing untoward occurring. But if that were so, then why had Boitelo come to see her?

“You are probably wondering what happened,” said Boitelo.

“I am, Mma,” said Mma Ramotswe. “I am not very sure where this is going. I thought I did, but now …”

“Well, I will tell you. I shall tell you what happened. One of our patients had a stroke. It was not a serious stroke, and he recovered very well. But he had a stroke. And he was one of the ones with high blood pressure.”

Mma Ramotswe nodded. “I have heard that this is a danger from high blood pressure.” She shifted in her chair. That was why that doctor had told her to lose some weight. He had talked about heart problems and strokes, and it had all made her feel most uncomfortable. What use, she wondered, was a doctor who made people feel uncomfortable? Doctors were meant to provide reassurance, which of course made people feel better. Everybody knew that.

“Yes,” Boitelo went on. “High blood pressure can lead to strokes. And this patient ended up in hospital for a few days. I don’t think that there was any real danger, but the doctor became quite agitated about it. He asked me to get out the patient’s records and he kept them with him for a while. Then he gave them back to me for filing.”

“And you looked?” asked Mma Makutsi.

Boitelo smiled. “Yes, Mma. I was nosy. I looked.”

“And did you see anything unusual?” prompted Mma Ramotswe.

Boitelo spoke slowly, seemingly aware of the dramatic effect that her words were having. “I found that the figures for a blood pressure reading had been changed.”

A large fly landed on the table in front of Mma Ramotswe and she watched it as it took a few steps towards the edge. It hesitated and then launched itself into the air again, its tiny buzz just audible. Boitelo had been watching it too, and she swatted ineffectively at it.

“Rubbed out?” asked Mma Ramotswe. “Were there marks on the page?”

“No,” answered Boitelo. “There was no sign of that. It must have been done very skilfully.”

“Then how could you tell?” challenged Mma Makutsi. “How did you know?”